As with anything that you wish to have paint stick to, remove all of the dirt, grease, and grunge from the stock. If the surface is glossy, sand it and re-clean. You can then paint the stock with spray cans of flat, earth-tone paint that you can find in most well-stock discount stores (I found a line of camoflage paints from Krylon at a local Meighers) or from Brownells.
On the ones I've done, I simply sprayed the stock with a base coat of olive green and then sprayed angled and random stripes of light tan and dark brown. Don't skimp on the light color(s). If you get the stock too dark, it won't hide well. I never tried to replicate "woodland camo" or anything fancy. I just broke up the outline.
The "Spray Grit" product that Brownells sells can be sprayed onto the areas of the stock on which you want some extra traction.