I recently did a faux wood finish on some vac-formed grip covers for my Grordbort raygun entry:
It does take some practice, but the basic method is to paint your base coat (a lighter brown color) with a gloss enamel paint. Once that dries, you have to make a glaze. In this case, I used a dark brown acrylic paint thinned with blending gel. You may also be able to use real wood stain and polyurethane, but always test it on the base coat first to ensure compatibility. Then you use a coarse bristled brush to create grain patterns. Once that dries, clear coat the whole thing and you're done..