Mount the pad with the existing screw holes then scribe a line around the base of the pad (use a scribe or a fine tip marker). That way you can see what to take off. Then, if there is a lot of material to remove, use a belt sander with a coarse grit that is clamped down to take off the majority of the excess. Then switch to a finer grit to get closer to the scribed line. Keep in mind the angles of the stock so it blends in correctly. After you get close, take the action out of the stock and remount the butt pad. Put masking tape on the stock so that you don't scratch it and use a 1/4 sheet sander (or similiar sander) with a fine grit to remove the rest of the butt pad until it blends in with the stock. There are jigs you can get (Brownells, etc.)that make this easier (getting the angles correct, etc.) but if you don't do this regularly like a gunsmith would, it probably isn't worth the cost (even though they don't cost a whole lot).