Having been asked by Confuzl for advice on how to play as a solo ritualist, I’m posting the (very long) full reply here. This is more focussed on archetypes than on the mechanics of casting - which Iulian has summarised very nicely in another thread - though the last section touches on that as well, given how important choice of Realm invariably is.
The Brass Coast are open to magic, enjoying the adventure and wealth it can bring.
- Hakima are spirited magicians, with a distinct Sufi and Arabian Nights influence. They’re the Brass Coast’s dedicated ritualists, who mostly perform using dancing and music, and tend to get involved in guiding and advising their tribes. Night and Autumn seem to be especially strong. You can do this alone but it’s probably easier as a group; on the other hand, I know that there are hakima covens active and recruiting.
Dawn is a high-fantasy nation where witches and wizards are a powerful and long-standing part of society. They are one of the nations most open to dealing with the Eternals.
- Witch weavers are the serious, dedicated magicians vaguely along the lines of Morgan le Fay, Circe, and Galadriel. They’re somewhere in between nobles and yeomen, and are known for meddling in Dawnish matters and generally poking holes in the social system while being very big on True Love and stories ending up how they should. The most common lore is Summer. Most weavers work in groups (cabals) but it’s not hard to join one in play, and there is an./*:m]
- Guisers are yeomen who create magic with drama, being very much the colourful, slightly disreputable travelling entertainers of mediaeval fantasy. They mostly lean towards Night lore. There aren’t many guisers (if any) in play right now, and it’d be a bit tricky as a lone ritualist, but you could do it with solo plays, puppet shows, or some other form of imaginative performance./*:m]
- Enchanters are Earls (leaders of their own noble Houses) who are also ritualists - a force to be reckoned with in both the magical and political arenas. They might master any Realm although Summer and Autumn are both likely choices. Playing a solo Enchanter is not the most straightforward option, but could be worthwhile if you want to play a political magician or to attract other Dawnish characters to your leadership through Tests of Mettle.
Highguard tends to be suspicious of the Eternals, but has a strong magical tradition including some of the Empire’s finest magician-priests and necromancers.
- Magisters are Highborn ritualists, typically with priestly training and/or a focus on the dead. They are your archetypal sombre, robed sages. They might work alone or in covens, using a range of methods including recitations, bells, drums, and slow movement to perform rituals. They tend to specialise in Winter magic
- Archivists are the explorers and scholars of Highguard, who might be anything from cloistered academics to Indiana Jones-style adventurers. Only some of them are magicians, but they could make good use of divinations from the Night, Day, Winter, and Autumn Realms.
Imperial Orcs have an instinctive, primal approach to magic, drawing on the voices of their ancestors.
- Oathwrights are the predominant form of Imperial Orc ritualists, deeply interested in bonds and how they interact with the idea of worth, as well as in dealing with Eternals. Imperial Orc ritualists are often also shamans. Because of the very social nature of orcs and their rituals, this might be difficult for a lone player, but does offer quite a different style from the human nations.
The League see magic as a valuable talent and a way of gaining money and influence. Magic, business, and performance are intimately linked.
- Mountebanks are street-magicians and tricksters - think of Clopin from The Hunchback of Notre-Dame, or Vinculus from Jonathan Strange & Mr Norrell. They can practice any Realm, but many lean towards Night or Autumn. This is ideal for a solo character as mountebanks often work alone, though they may equally often team together for more powerful rituals.
- Troupes are the upscale, organised magical covens of the League, using dramatic and musical performances to create magical effects. Again, they can get involved in any Realm, but Autumn is especially well represented. This style has a lot of variety but is for obvious reasons more difficult to get involved in as a solo character.
The Marches can be suspicious about sorcery, but have a number of strong magical traditions, and their Landskeepers are pivotal to the fertility of Marcher farms and orchards.
- Landskeepers support the traditions, wealth, and fertility of the Marches, with a strong influence from Pratchett’s witches and mediaeval cunning folk. They’re a lot less partisan than most Marchers and tend to work across household (or even territory) boundaries. Landskeepers mostly work with the ‘seasonal’ Realms, especially Spring, Summer, and Autumn. They might work individually or in covens, but either way tend to be good at cooperating for the good of the land./*:m]
- Mummers are the counterpart to Dawn’s Guisers, and are similar in operation although they tend to use more improvisation and less subtlety. Like the Landskeepers, they tend to specialise in the ‘seasonal’ Realms./*:m]
- Threshers embody the Wicker Man-style distrust that runs through Marcher culture. They aren’t always magicians - what defines them is that they are dedicated to hunting down sorcerors who misuse magic and bringing them to justice, whether helping law enforcement or as ruthless vigilantes. A magically inclined Thresher would get most benefit from Day, Winter, or Night rituals./:m][/ul]/:m]
The Navarr lost their ancient empire of Terunael in a magical cataclysm centuries ago, leaving their ruined cities in the grip of the ravenous Vallorn. Now they use magic as another weapon to fight back and reclaim their birthright.
- Vates are magicians who have sworn an oath to serve the nation when dealing with Eternals and their Heralds (and are the only Navarr trusted to do so), and help in the fight against the Vallorn. They have some druidic influence but maintain their own distinct style. They might deal in any Realm, although they often have some knowledge of Spring. Because of the often nomadic nature of the Navarr, this is a decent choice for a solo player - you can join a coven later or continue working on your own.
Urizen is a land of magic, where magic is king. Most people in the nation know at least some magic, and ritual ability is so highly regarded that voting rights are granted based on citizens’ lore skills. They are also one of the nations most open to dealing with the Eternals.
- Magi are the political magicians of Urizen - think of the Jedi Council, the Maesters from Game of Thrones, or the Assembly in the Riftwar books. They aim to build influence with mortals and Eternals alike, making themselves pivotal nodes in the Net of the Heavens. A magus might master any Realm, but Autumn is particularly fitting. Gathering political influence as a lone character is a challenge, but entirely possible, and being a pure ritualist in Urizen gives significant clout in elections.
- Stargazers are magical researchers interested in the fundamental truths and nature of Creation. They are the experimenters and improvisers, who might be inspired by any archetype from Plato to Victor Frankenstein. They can and do work in all six Realms, but at present Night, Day, and Spring have especially strong active research groups. This is very much open to solo players, although if you want to do more powerful rituals you should consider joining a coven in play.
- Seers are specialist chroniclers and researchers of the world around them, interested in recording every fact and detail. They can be calm historians and commentators or intrepid investigators. Most of them are masters of Day magic, but other realms such as Night also see use in their divinations. As there are some good low-level divinations, this would be easy to play as a solo player.
- Torchbearers are a loose political faction that overlap with the Seers. They are committed to honesty and transparency, and might be by-the-book inquisitorial types or Wikileaks-style muckrakers. Not all are magicians, though most are, and make good use of Day and Night rituals.
- Architects are traders, economists, and investors. They use the Net of the Heavens as a guide to how and where to invest their resources for the greatest return and benefit. An Architect who is also a magician would most likely use Autumn rituals, but might also benefit from Day lore. This is a possible (and under-represented) option for a starting player, and would work well with a fleet resource, but a solo character will struggle to match the buying power of established groups.
Varushka is a land haunted by supernatural horrors, where magic is both a deadly threat and a powerful defence against the monsters that lurk outside.
- Volhov are wandering meddlers in the spirit of Gandalf, Granny Weatherwax, and Baba Yaga, who have a reputation for interfering with the lives and destinies of others. They are occult problem-solvers who both defend against the terrors of the night and strike back against their enemies. A volhov could use any Realm, but Winter and Night are especially fitting. As volhov are often solitary this is an excellent choice for a solo player.
- Wise Ones are the guiding lights of Varushkan society, standing as shrewd and determined foes of the Varushkan monsters. Not all are magicians, but if they are they should be capable of supporting and advising the community and of beating the monsters’ snares. This is entirely possible as a solo player, but note that being a Wise One is not a formal position - it’s something that you have to earn by living up to others’ expectations.
- Cabals are groups of Varushkan magicians who gather for casting powerful rituals and often other, deeper purposes. They might resemble fairy-tale warlocks or the Three Witches in Macbeth. As they are by definition group enterprises, this is difficult to do as a solo player, though a volhov or wise one might easily join a cabal in play.
- Stzena are singers, musicians, and night-watchmen. They might have any mix of skills; Stzena who are magicians could use Winter magic for warding, Summer to strengthen them in battle, Spring to heal others, or any other sort of lore. If you like a bit of music, whether or not you’re a virtuoso, this could be a fun and viable plan as a solo player.
Wintermark, as a land forged from three tribes, has three distinct magical traditions.
- Runesmiths are overwhelmingly Steinr, resembling Germanic or fantasy dwarven magicians. They see magic as a weapon in itself and as a way to supplement their mundane weapons. Many focus on Summer or Autumn magic for this purpose, although some use Spring magic for healing. This is definitely viable for a solo character as there are some strong, cheap combat buffs, especially in Summer.
- Icewalkers are almost all Suaq, based on Inuit and Siberian shamans. They aim to make the best use of their wits and perceptiveness, cleverly solving the problems they face; they are known for dealing astutely with Eternals and for their invocations of animals and natural forces, typically focussing on Day magic. Again, this is a good option for a solo character.
- Mystics are mostly Kallavesi, based on the shamans and visionaries of the Kalevala and the iron age peoples of Europe. They have a reputation for serving as wise advisors and diviners; not all use magic, but most have a knowledge of Night lore. Also a good option for a starting character./:m][/ul]/:m][/list:u]
So that’s a lot to choose from! My advice would be that the following are the most worth considering:
Ideal for a solo ritualist: Mountebank (League), Volhov (Varushka)
Also good: Hakima (Brass Coast), Weaver (Dawn), Magister or Archivist (Highguard), Landskeeper (Marches), Vate (Navarr), Stargazer or Seer or Torchbearer (Urizen), Wise One (Varushka), Runesmith or Icewalker or Mystic (Wintermark).
A starting pure ritualist basically has two sensible build options - Lore 2 in each of two Realms, or Lore 2-3 in a single Realm. If you want to have other skills as well, you should still make sure you have Lore 2 in one Realm, and spend the three remaining XP on Battlemage, spells, combat skills, or anything else you can think of. The exception is if you start with Artisan and one rank of Realm lore, as you can then buff yourself with items and save up for more Lore.
The default personal resource for ritualists is, of course, the mana site. You will go through those seven crystals astonishingly quickly, especially if you’re active in Conclave. The other choice that might work for some builds is a fleet, as it’s very flexible and the only way of combining mana crystals with any other material income - this is very good for artisan-magicians and traders, and a fleet also can be buffed using Autumn or Day magic. It’s possible to make a ritualist build work with another resource, but that’s a little trickier.
On the subject of resources, certain items can be very useful to a solo ritualist. Here’s a (by no means exhaustive) overview:
- Volhov’s Robes allow you to cast rituals with a coven as though you were part of it. This is extremely useful, not least as it allows you to switch between covens, and you don’t have to worry about coven oaths./*:m]
- Wands, rods, or staves that allow you to cast a spell as if you knew it. All of them are cheap and take up an “armament” item slot, which you probably won’t be using anyway. If you want to do spells with your personal mana but are pushed for XP these items are a godsend./*:m]
- The various items (generally staves and rings) that boost your Realm lore. Expensive but very, very useful. Masks aren’t so good but you might be able to find a use for them./*:m]
- A set of potions (one for each Realm) that give single-use boosts to Realm lore. Good for emergencies or to give you that extra push, but obviously you need a new dose each time./*:m][/ul]
The most powerful ritual you can cast solo in a given Realm has a magnitude of twice your Realm Lore level, or twice your level plus two if you’re in the Anvil regio, if you’ve mastered it. Otherwise you can cast at a magnitude up to your Realm lore level. What this means is that you should consider what you want to do and choose Realms to suit. (Note that when buying skills, you can automatically gain mastery of up to two rituals per level of Realm Lore, but you don’t have to - you can leave the “slots” blank and save them for later, which can be very worthwhile if you’re just getting into the game.)
So, let’s look at the rituals that a Lore 2 ritualist can cast solo (Magnitude 2 to 6) in each Realm:
- Spring - The best healing rituals by a huge margin, but the other ones are highly circumstantial. Indispensable for healing, not recommended otherwise unless you have a use for the circumstantial rituals.
- Summer - No fewer than seven very solid combat buffs, and a couple of minor resource buffs, although it’s not great at anything that isn’t to do with fighting. Strongly recommended if you want to get involved in combat or work with other people who do.
- Autumn - A wide array of thirteen cheap rituals, including item-related divinations and some roleplaying effects, although most are very circumstantial. Recommended if want to do magic with bonds or items or if you think you’ll get good value out of one of the other specialist rituals.
- Winter - Has a few neat tricks, especially for dealing with dead bodies and curses and for destroying things permanently, but again most of the rituals are circumstantial. Recommended if you want to do necromancy or work with curses.
- Day - The best for divination, with three strong, cheap divination rituals. Also has some other fun effects including a decent combat buff. Recommended, especially if you want to do divination./
- Night - Fifteen, yes that’s right, fifteen rituals at Mag 6 or below. These are a real mix of effects with a leaning towards roleplay effects and playing tricks on people; it’s also got several ways to stop other people’s divinations and some basic divination of its own. It is, however, very bad at combat buffs. Recommended, especially if you want to do sneaky things.
In summary, have a think about what play style would suit you, then pick a nation, character archetype, and Realms to suit. There are endless possibilities by combining the options available to you. 
edited to solve rogue cut and paste; edited to add section on items
