Painting's pretty easy... There are a few threads with the step-by-steps. A general Google Search will get you a some real good paint/nail polish threads on XD-Talk (as well as a few YouTube videos), if you can't find 'em here.
If you're not sure about which color you want, you can experiment with Crayon (Crayon comes off easier).
Check this site. The technique is pretty similar for China Marker, Crayon and Paint.
White Lettering On Guns

I use Crayon and change the color periodically.