I have cut down maybe half a dozen wooden stocks for my friends over they years. I generally do it on a power miter-saw. Because the stocks are tapered and won' lay flat or square to the bed and fence, it works best to measure the distance you want to cut off from the back of the stock and scribe a line to follow. Then prop and shim the stock on the saw base to get the correct cut and then clamp it or make sure it's held securely.(otherwise messy and dangerous things can happen). Cuts that are off a little can be adjusted with a belt sander or better yet a table disc sander. Remember, you can always cut more off, but it's hard to put it back on after you saw it off.
Sometimes, depending on much is cut off, and how much taper the stock has, the recoil pad or butt plate will need to be sanded down to match the new contour of the stock.