If its steel, and there is a bit of rust… you will need to get that off. Or at least smooth it out, a toothbrush and WD40 is good for a little rust. Wire wool/scrubber for something a little more… entrenched. Just becareful not to scrub more than you need to. If you can take your strapping off easily… go for it. You dont want them getting paint all over them if it can be avoided. If they cant be easily removed… tape them up to death as to not get paint on them.
You will likely need a primer. Then a base colour. I would use a spray for these to get a nice even coat. You may even get both primer and base in one can. Spray in a well ventilated area. Careful of wind and other items (ive not totally done this before…) you can find paint all over the place if you are not careful.
Find your base colour, and give it a couple of coats of that. Make sure it looks like a uniform. Following the instrustions of the can.
If you want trimmings of certain colours, do this first. Using masking tape to tape over the trimming, then do your other layers over this. Its best to plan this out first…
Next is detail, if you want any. There are two ways you can do it, off the top of my head.
Find some special decal paper online, like the stuff you get for warhammer/airfix models. Put this in your printer at home, if you can… and print it. See if you can slide this on. This may cost a fair bit however, as its specialist paper/process. So you may want step two… which is how i do things.
Paint it yourself, get some nice and easy acrylic paints and some good paint brushes. You can get these online or model shops. Try not to buy games workshop ones… they cost a lot for what they are. Then get paining. Sketch ideas on paper… then do it again as a stencil. Use said stencil with masking tape to make the initial design. Then go for detail with tiny brushes if you desire.
Once you have done this, and you are happy with how it looks. Break out the spray varnish. Make sure its suitable for outdoor use. As you dont want it melting in the rain. Give it a couple of nice even coats, dont be tempted to do one coat for all… its better to do a few as it dries more uniform.
Take photos! i wanna see