This "basics" is for a fiberglass repair where a piece is missing. It could be used even if the piece isn't missing, but just cracked badly. What you do is thin the cracked area very thin so you lay the mat over the top just as you would in the description below referring to a missing piece.
NOTE: Please be sure to research and follow all safety precautions before working with fiberglass. When using resins you must have proper ventilation and an air supply, you must protect your skin -- and you must have airtight eye protection. The vapors alone from resins can cause blindness. Don't take any chances with your health. If in doubt, contact the supplier of the materials you are using, to be sure to understand the correct safety aspects of what you are getting into!
This is not a very difficult project. First you need to grind or sand with very course paper like 40 or 36 feathering the fiberglass out to thin it on the front and the back along the edge of the missing part. Don't thin it too much, at least not more than a half inch or so away from the missing part. At the edge you can go right down to a knife edge thin, but at about an inch or so you should let it remain pretty much the way it is. Taper it out to almost the original thickness within that inch. Just sand it down real good after that many more inches away on both inner and outer.
Next, cut your fiberglass MAT, not cloth but MAT. Fiberglass mat is the stuff that looks like it was shot out of a chopper gun with no particular patterns, just many many pieces of fiberglass strands laying over one another to form a "mat" of fiberglass. The cloth is the one that has all the fibers laying in perfect rows in a crisscross pattern. The cloth will ALWAYS show up later, the mat is basically the same as the car was made with so it works very well in patches.
You want to cut many pieces of this mat starting with a number like three or four that are very close the size of the missing part, even a little smaller. Then make a few that are a little bigger, then a little bigger then a little bigger. All of these should be in the area of an 1/8" to a 1/4" bigger than the last.
Next using 2" masking tape make yourself you "mold." Stick the tape on the back side across the missing area.
Now, mix up your fiberglass resin with hardener and using a little tray one of the smallest pieces of mat you have on the tray and pour a little resin on it. Using a short bristle brush "poke" at it with the ends of the bristles "pushing" the resin into the mat. You will see it become transparent when fully soaked with resin. At that point lay it down in the middle of the missing area. Then soak another piece and then another when the middle of the missing area is covered (you may not use all the small pieces you have cut) start going out with the larger pieces. You don't want the middle to be too thick or you will be grinding it all off later. So right as it is getting thick you move to the next size larger. All the while you are "poking" at the new pieces lightly with the ends of the bristles to push all the air out from between the pieces of mat, forcing in the resin. After you have covered the missing area and gone out some onto the feathered out existing fiberglass allow it to FULLY cure.
Fiberglass takes a long time to fully cure, sometimes hours. Give it time, if you start sanding on it all you will end up doing is peeling it up on the edges and it's adhesion will be compromised. After it is fully cured remove the tape from the back and lay some large pieces of resin soaked mat across the back of the repair going out on to the existing fiberglass a good couple of inches. It may not be too pretty back there, but it will usually not be seen anyway. If you wanted to you could smooth it off and even apply somebody filler over it and smooth it out real nice as well.
Once it is fully cured you can sand the outside to shape. When you cut into the mat you will likely have many small air bubbles and pin holes, not a problem. At this point you treat it like minor dent in metal, a coat of polyester body filler (bondo) should take care of it. I personally like to use Evercoats "Everglass" right over the repair and then a skim coat of "Glaze Coat" polyester putty to finish it off to perfection.
Tip 1: Get disposable latex gloves!! And it is advised to use throw away brushes and mixing containers. The small cost is well worth the savings in work.
Tip 2: Get cheap brushes and cut off the bristles about half way so they are stiffer.
Tip 3: Use NEW hardener. The hardener only has about a six months shelf life. Sure, it will last longer but six months is the "safe" shelf life, stick to it and you won't have a problem. Use an old one and find out how much fun it is to scrap off semi cured fiberglass (ask me how I know).
Tip 4: Use the gloves when sanding as well. If you can use a long sleeve shirt taped to the gloves and pulled up around your neck high. Wear a good particle mask! Fiberglass dust is very dangerous and irritating to the skin. After sanding go take a cool shower and just let the water run over you to wash it away before any scrubbing.
That is all there is to a fiberglass repair.