>>6112345/tg/ here. Heres how you paint, well...Most plastic. Models and the like.
First, a layer of primer. You're gonna need primer. If you don't have primer, get some primer. White primer if you want your colors to be brighter, black for a darker tone. In your case, you probably want white primer.
Next, you want to thin your damn paints. Always thin your paints. You know why most paint jobs look like shit on mini's and toys and whatever? Because they didn't thin their damn paints. Just a bit of water will do, usually, depending on the type of paint.
Next, you're going to want to do several thin coats of paint. It's gonna take time, it's going to be boring, and you'll probably come out of it with a sore neck. But if you don't want your thing looking like shit, it's the only way to do it.
Finally, you'll want to seal the thing. Because guess what? Paint chips. It rubs. It's generally all around shit! The reason paint on factory toys doesn't usually scratch or chip so easily is because that shit is industrial grade and highly fucking toxic, and applied by machines that won't, y'know...Die. From being exposed to it. Regular modelling paints are not that tough, so you need to seal that shit. Matte or gloss, it's your choice. I'd recommend two coats in either case.
And make sure you don't get any paint or sealant or primer in the joints or on the joint pegs, because that'll fuck things up in there, making pegs too thick or joint holes too narrow, etc.
Good luck, Anon.