what would you tell anyone thinking of starting one up?
Spring magic is best magic, be prepared for “banter” from people about Vallorn and Briars with witty comebacks 
Think about whether you want to bring the UNSTOPPABLE HEALING WAVES (See below) on to a battlefield and make yourself every fighters best friend. 
Some risks entailed.
What are the cool rituals ,
Behold my extensive wisdom as a militant ritualist To be honest its mostly stuff to avoid, though dont forget you can change your ritual masteries!
Blood of the Hydra: Cast it once, to be honest, Physiks are usually on broken limbs like wasps on jam, take it if you have a spare slot somehow.
Hands of Sacred life Far and away my most cast ritual, dont forget people will often pay you if you ask nicely and/or guilt trip them or their combat leaders!
It only gets more efficient as you up the power but beware enemy action as you are squishy and super easy to shoot. I have however performed this ritual lying on the ground with arrows flying around the place, all fun times
But yeah, ideally, pick a lull in combat and lure over a group of wounded slightly behind the line, talk to your captains and generals if needed to time this, trying to work amongst a mass of stressed out physiks doesn’t really work and effort gets duplicated. When in doubt, run around slapping your targets on the back shouting YOURE FULLY HEALED because half the field does not understand esoteric ritual magic 
Turns the Circle For legal reasons i have certainly never cast this
Hearthfire circle Almost always overkill, rich people might pay you to hang around with this on standby. Probably wont though, curse you sovereign specific potion!
Call down Lightnings Wrath I have genuinely felt OP using this at player events in combination with a mana restoration aura. Strikedown for 1 mana is brutal and dont forget it works through weapons so just tap the tip of egregious polearms and make them kiss the dirt. Win almost any solo fight! Pricy if you arent casting it solidly though, so invest in some mana potions or similar for battlefield use.
Churning Cauldron of Bravash Unless you find some weird rare herb, its awful. I mean unless someone is paying.
Merciless Wrath of the Reaver Solid earner if you can find someone who does not know some absolute git casting it for free 
Rampant Growth someone paid me to master this, i have never cast it.
Blood and Salt Another solid earner, works with some fleet actions so go out and promote if one of those is going on!
Unending cascade of Bloods fire I mean just say it, its badass and the Vallorn absolutely MELT when a couple of casters with this hit them.
Thats about my limit as a habitual solo caster, but i should mention Curse of Gangrenous flesh is absolutely brutal and threats of its use can terrify people into doing things for you if you put your thumb over the Magnitude in your grimoire while showing them. Allegedly.
Please try to avoid war crimes when casting the big curses
what kind of items would you recommend they make or look out for, what potions?
I am mostly a battlemage but the +Lore staves are obviously good as are the various potions that boost your castings. They are all a bit pricy but can push you to a new ritual that might be very handy. Let your maths folk debate it. You need someone who loves maths incidentally. ASAP. Best coven asset there is.
I tend to wear unstoppable granting mage armour because arrows are a thing.
Would you recommend any particular Magical Traditions to them?
Blood magic is the true hotness but i also dabble in Theurgy because Highguard and frankly, fake blood is super awesome on battlefields but juggling a weapon, fake blood and situational awareness is sometimes perilous!
Essentially its the Pratchett quote, you can do it with 2cc’s of mouse blood in a pinch
What kind of shenanigans do Spring Covens get up to?
Most of my casting is battlefield healing, i sell buff rituals to fund my lavish healer lifestyle (lolololol) as mentioned above though it tends to be loyal return customers and friends despite me telling anyone who will listen that Call down lightnings wrath is life changing.
Which Eternals should they talk to, oppose or at least have frosty silences with?
most of the Spring Eternals tend towards being antagonistic, because they are super elemental and the things they represent tend to not go with Empires
The ones that arent outright enemies are still working against us long term, that doesn’t mean we cant use them, just dont forget it.
Dont forget Llofir invaded the Empire recently!
How do they work in your particular nation’s magical traditions and Hearth Magics?
Highguard! My character is an Unconquered rather than a Magister for various reasons so i cant comment too closely on that.
Hearth magic wise, Cleaning goes very well with blood magic, assuming you have not bought super cheap fake blood that WILL NOT DIE.
Bells, chimes, Cowls and veils are all great ritual props used correctly, props are always a bit handy to fill space and look great!
A Journal can also double as a super handy grimoire and rules reference to all the rituals, pre calculate as many numbers as possible and scrawl them in the margins to save time! I have literally printed out all the rituals, potions and handy things and stuck them in mine to get the wording exact, then, as mentioned, scrawled IC notes, maths and ideas all around them. Add arcane projections, ideas, observations about barbarian magic to taste.
And the most important question, what is the aesthetic for Spring Magic this season? What is the new green? How should Spring mages accessorise?
Black is the new Green, with a white secondary Green is out baby! Get yourself a staff and some mage armour to crack heads with lightning and stop the first barbarian with legs chopping you up, then grab every ritual prop your hearth magic recommends, then a few more, oh and a good ritual knife, also handy for cheeky barbarians.
Then some more knives. You can never have too many knives!
Never forget, Spring magic is best, Briarism is welllll under 1% as a side effect.
(This has actually happened more than once).