Using a circular saw so you can control depth. Then you'll need to grind the edges so you can taper in the fiberglass patch. I think 12:1 overlap is the accepted standard for the new layup patch to adhere properly. I'd use epoxy or vinylester rather than polyester because it's stronger, has a better secondary bond, and it's below waterline so you'll definitely paint not gel coat. Some people would advise against using epoxy for a patch because it will create a hard spot that won't flex as much as a polyester hull, but this is cored so I don't think that's as much of an issue. Even so, vinylester is the answer to that issue I'd you were using solid layup.