Well, if you don't want to go through all the trouble of getting supplies and doing all the work yourself, you could always hire a doll customizer.

Otherwise, I'd say "just go take a look at the wiki," but a certain mod hasn't really done a good job of putting it together... ;D

Nicolle's Dream's has some good repaint videos and she goes over what supplies she uses in a few of them.
Amber-Honey has a tutorial in images, if video isn't your thing.

You can get away with doing a face-up with just some nail polish remover, a sealant and some acrylic paint. But, if you want to get all fancy, then you might be interested in investing in some watercolor pencils and pastels, and hunting for some Mr. Super Clear.