I have never done one myself, but the basics are:

Flip the boat over (keels up)

File or grind the keels a bit flatter as a base for the new glass to stack onto.

Take 2" glass tape (woven comes on a roll).

Use laminating resin and wet out a layer of cloth and resin. Stack up 6-8 layers one at a time. Squeegee down tight between each layer.

Let harden (a little) till what we call "green" not yet hard. Use a utility knife or longer blade to trim / carve off the excess glass as best as possible. Would require a lot of grinding if you wait too long.

Shape with a rasp or angle grinder when hard.

There is a keel-shape template shown in the support area of the Hobie Cat site: Keel Template

The guy who did them in San Diego would add a clear coat or gel coat only if desired by the customer.

If the keel is split already you have to grind down the gel coat on the sides of the hull on either side of the keel. He would lap the first layer of glass or two over the keep and down the side about an inch to laminate the keel together first and then stack up some layers.