I've never done it, but i'd paint it like i was painting a body. This is just what i'd do, whether it's the best method is another thing.
1 Sand the pickguard with 320, you dont need to sand much, it's just to "rough it up" slightly, if it's too smooth the paint will have trouble sticking.
2 Spray it with primer, just two or three thin coats will do it, so spray, leave for an hour, spray ect. When it's dry (at least four hours), sand flat with 800 wet, then it's time for the colouring.
3 Pick your first colour, spray thin coats, i'd spray coat after coat (leaving it to dry inbetween) until the primer had gone, then spray two more. Again, when this has done, sand with 800 wet to flatten it out if it's bumpy.
4 Then do your patterned stuff, take your time, if you're not happy, leave to dry and sand back to the first colour coat with 800 wet and start again, when you're done, pick the next one and keep that cycle going until you're happy, then clear coat it.
5 On guitar bodies, you spray three coats a day, with at least three hours drying time, for three days, that's for a solid finish. Your pickguard won't have the abuse a body has, so just try three coats, that should be hard enough for a pickguard. Then sand with 600 800 1000 1200 and 1500 till it looks slick, and then maybe polish it with some rubbing compound.
Again, that's what i'd do, it's not cheap if you're just doing one pickguard and you have to buy all the stuff, but if you want a one-off custom pickguard, then price isn't considered anyways.
Before you try it, wait to see what someone else like robert original has to say about it, he might suggest an easier way for you.
