If doing this yourself and first time, you trim a recoil pad by screwing it on, oversize and all, placing a couple of layers of masking tape on the stock forward of the pad, then trimming the edges of the pad on a belt sander. The tape protects the stock finish from any little glitch with the sander. Continue the stock lines on the pad. Most common mistake is to sand bottom of pad parallel with the barrel rather than in a line with the bottom of the stock. Looks like h--l. It takes practice to get it right, which is why smiths charge for the job. If you want to do it yourself, better practice on an old scrap stock.