Your best bet would be to get wizardofflight's Webley templates for the sizing of the stock. Get some wood (Mid-grade hardness...don't use pine! It's way too soft and will dent, ding, scratch easily)
Use the templates as a guide to rough out the shape with a knife, chisel, preferably a dremel, and then smooth down to final contours with sandpaper. Start with high grit (100 grit) and work your way up the line in grit depending on how smooth you want it to be (probably up around 800 grit).
The most important thing to remember is patience. Use real gun stocks for reference, also.