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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. 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.