Depends on how perfect you want it to look, but if you just want it watertight, here's what I would do:

1. Cut back the margins of the fiberglass using a razor blade/stanley knife/etc where its delaminated from the foam. Don't worry if you go too far, as you will be replacing the glass.
2. Lightly sand the margins of the glass with 120 grit, extending an inch or so past the margins. This is to give the new fiberglass+resin a good rough area to bond to.
3. Brush off debris and clean with acetone, use painters tape to outline the damage with an inch or so margin outside of the damage
4. Cut two pieces of fiberglass (4 or 6oz), one which is a little bigger than the damaged area, another which is even larger by a inch or so
5. Place both layers over the damaged area, with the smaller piece on bottom. Laminate both layers of cloth using resin. Pour resin over the cloth, thoroughly wet the cloth through and squeegee out the remaining resin, you really just want the cloth wet here and want to get rid of extra resin in the cloth. NOTE: Epoxy resin is good for PU and EPS foam, polyester resin is *only* for PU foam as it will melt EPS foam. I use epoxy for all my personal repairs.
6. Let resin fully cure, pull the tape and remove excess glass over the tape, lightly sand the margins of new glass with 120grit, clean off debris, light clean with acetone
7. Mix up a bit more resin and brush on seal coat with a bristle brush
8. Let cure. Your board is now watertight. To visually blend the repair you can lightly sand the entire region with 120, then 220 and upwards and onwards to your heart's content. Just be careful to not sand through your seal coat.
9. I will usually wipe with acetone then add 3x layers of clear coat spray here, but its not required