I am 90% there with David with minor changes.
If the hull
is thick enough I would bevel the inside and the outside of the hole. I would shoot for 7-10X the thickness. That is if the hull
is 1/2" thick shoot for a 4-5 inch bevel all around the hole. for small thru hulls (2 inch diameter or so) I would reduce the bevel some.
With a double bevel the plug
overlaps original material on both inside and outside. The 1/4" increasing patches is the right ticket. I would work the inside and then work the outside. I would also prepare an area an inch or two beyond the bevel and the top layer of fiberglass overlaps the whole thing thicker than original hull thickness and then gets ground/sanded off. I don't prefer to have the plug "thinner" than the hole and filled with raw epoxy as epoxy is non-structural and I prefer fiberglass at least as thick as the original hull. On the inside you do not have to grind/sand back to original surface - Clean it up smooth and leave it a little thicker unless you really don't want the repair to show and are going to also finish it on the inside.
When you grind/sand the plug back to flush pay attention to what happens - If "weave" breaks loose and stands up free, you didn't penetrate the resin into the fibers well enough. If there are minor imperfections after you grind/sand back I would fill with a microballoon and sand /prep for paint.
West System is easy to use and I have found it reliable. I had to "rebuild" my pulpit area after hit and run collisions while my boat was moored. For smooth surfaces mat weave is fine. I had compound curves and found chopped mat the right ticket for me.
Fiberglassing is chemistry. Mix the parts
per spec. Too much activator and the mix will get superhot and pot life will suffer. Mix a small batch exactly per instructions and practice on something - get used to pot life and how much time you have to work it. A larger batch in a deep tub will activate faster as well - its got to do with the chemical reaction.
Oh final two thoughts - when you are ready to lay in teh fioberglass make sure everything is superclean with thinner. No oils or debris. Get a box of surgical gloves and wear them. I am a slob and can't keep my fingers out of stuff. Once you contaminate your gloves strip them and put a new pair. As the epoxy starts to activate things get very sticky...