Put at least 2 layers of masking tape all around the stock prior to trying to do anysanding on it.You don't want to mar the finish on the stock.
You need to make a scribe mark all the way around the new pad to give you a place to rough sand down to,so go ahead and install the pad now and make the scribe mark on it.
After making the mark around it,I gennerally take them back off of the stock to do my rough sanding.I start out with 60 grit for a little bit,not heating the pad up to much as I go.The hardest part is following the contours,but if you go slow you should be alright.I then switch to 100 grit working more and more around with it.Go slow and don't heat the pad up too much.This should take you down to your mark you made on the pad to start with.Depending on the thickness of the tape you put on your stock,you should be pretty close to having it done.You will need to put it back on the stock for the final sandiing.This is where you'll have to be very carefull when doing it.I switch over to 120 grit then 200 grit and do this part by hand using a sanding block.Sand down thru the first layer of tape and check the finish of it all around touching up where you need to.Remove the tape from the stock and check it again.If you don't want to sratch the finish on the stock...take the pad back off before continuing sanding......If you are going to refinsh the stock the just take the tape off of it completely to do the final sanding and it will meld with the stock a-lot easier. After all the sanding is down,I usually wipe some Rem Oil on it(lightly) and wipe it dry.
Hope that helps
Mac