I use a couple layers of masking tape and a pen to mark the "cut to" line. Cut shy of the line, I use a back saw, then true and cleanup with a disc sander.
~ Plastic melts easy, try not to generate much heat.
~ If the stock is currently flat, maintain a parallel cut.
~ If the stock is currently curved, maintain the same pitch - important.
~ A little goes a long ways, an inch is a lot, cut a shorter length to test if you're not sure.
~ Make sure there are no bolts or tubes in the stock that will interfere.
~ Make sure the replacement pad will actually work and are available before cutting.
~ Seal the wood when you are done cutting and sanding.
~ Make sure there is a place to use the pad screws, many plastic stocks are a pain.
~ Sling studs may interfere with the pad screw, plan for contingencies.
~ The holes for the old pad/plate probably won't work.