I don't know the "proper" way, and have always hated to fit recoil pads. But we do alot of them over the years in our shop. I remove the old pad, install the new one, trace the inside of the pad with a scribe, or sharp pencil, remove the pad, and sand it.
You could use a belt or drum sander. You will have to keep the factory angle or it won't look right. You may want to put it back on a couple of times to reconfirm the fit. Go slow, and remove a little at a time when you get close. Use a Scotch Brite pad to remove the sand scratches when you have a good fit. That is about it! You will be covered with black rubber, and will hope you never have to do another! Good Luck!!
By the way, If you have to refinish a stock, it makes it alot easier, just leave it on the wood. You only have to remove it when you stain/finish the wood. This is very easy, and fits the best! I wish they were all that easy!!
CJB