Fiberglass resin is really really easy to use. Just follow the directions on the can.

Just make sure the activator is fresh, there might be a date on it. Comes in a small tube or small bottle.

Just pour a small qty of resin into a paper cup. Put the recommended number drops of the activator to start the chemical reaction. The more drops of activator you used, the faster the resin hardens (to a point).

The resin in the can will last a long time, it has a good shelf life. It's the activator (small tube) that goes bad/ less effective over time.

Stir the resin & activator throughly in the paper cup. I use a popsicle stick. Get a paintbrush (appropriate size for the work area) and brush on the resin mix onto the damaged area.

You can put fiberglass cloth (cut to size with scissor) over the resin. Use the brush to position the cloth patch and work the resin into the cloth. When the cloth goes transparrent, then its properly impregnated with resin. Brush in resin until the the cloth goes transparrent. Then you let it harden and repeat the process, if you want to add more layers of fiberglass cloth.

The resin will settle with the pull of gravity. If you can position the damaged area towards the bottom (flat), you can let gravity even out the resin over the damaged area.

You can clean up the brush and mess with MEK.

For small damages and cracks, you can also try super glue...