Hi , I am far from a gunsmith but that has never stopped me from pretending , I took a rem 870 and cut the stock and refit the factory recoil pad
I removed the pad and measures what I wanted removed from the butt stock and wrapped that area I was going to cut in masking tape to prevent splinters , then I had to measure again and drew a line on the tape where I wanted to cut , borrowed a table mounted circular saw if thats the correct term and cut it , worked out nice.
then I remounted the recoil pad and the bottom "toe" of the pad hung low , I once again borrowed a tool , this time a table mounted disk sander , worked out well also.
A word on the disk sander , coat the pad area to grind down with vasoline , use a fine grit , take your time and don't heat up that rubber pad