Also once you complete the sanding before you start glassing check the edge of the fiberglass to see if it's loose. I would probably go wider but plan on sanding through the edges of the fiberglass to smooth it out. Perhaps however you shouldn't make too much of a job of it since it is your first repair. In my mind If you have a small strong point like a glob of resin with fiberglass in a hole then if something hits that it will transfer energy to the surrounding area making it crack there.

I don't mind cosmetic defects as long as the end result is strong. Then you need to replace the damaged foam with a product first before doing the glassing. Also you need to remove damaged foam for the same reason as the damaged glass. If you don't sand away a bunch of resin around the ding in the area of fiberglass overlap there will be a bump and it will be heavier. However if the youtubes say to apply them at the same time go ahead. I am not that familiar with epoxy resin but have done a few repairs myself and I would do the two layers of glass separate and overlapping. So remove the loose glass and let it sit with the hole at the low point for a week or three in a warm environment. You have to dry the lesion out if it was in the water with that ding. This is an epoxy board? Do you know what kind of foam? I would probably take my board to have someone else repair a ding like that especially if the board is still fairly new because if you botch it you have to live with that for the rest of the time you have this board. If the fiberglass came loose from the foam it will create pocket of air that will be easily damaged and maybe allow water to leak in. You want to be sure you get all the loose fiberglass removed which might be a huge amount given the L or T shape of the ding.

How to shoot crack resin hit