What does *he* look like? Is it a decal job? Paint? Sculpting? Have you checked out the KAM and MCN sites? Brickshelf has some good stuff too, but "clone trooper" is incredibly broad, so I don't know what you are looking for.
MCN is a pretty good place to look for help threads or instructions. (Found by googling "custom minifig.") Specifically, they have a how-to section. Here's he one all about painting. THIS came up after searching "minifig custom painting helmet"