The trick with fibre glass is too work clean and imagine that the resin is like gold so use as little as possible, acetone is the only think which will wash the stuff off brushes etc while still wet.
without looking at the crack i can only guess at the best way to fix it.. start off by taping the front of the fairing accross the crack to get it level and in the correct postion then work from the back.
1. cut medium weight chop strand matting to the correct size you are looking for about 4cm band over the crack. if you are working into a corner or a complex shape it is far easier to use tissue.
2. cut a paint brush down so its hairs are about half length a brush about 3/4" is normally most usefull.
3. mix the lapup resin with the hardener. the stuff from the car accesory shop is okay but expesive in large quantities.don't be tempted to use too much hardener as the resin will set with in a few minutes
4. paint the area with the resin and press the matting on to the surface then use the paint brush to stipple (press) extra resin into the surface.
one or two layers of matting should be aduquate.
5. when cured the repair can be cleaned up and any excessive matting can be cut off with an angle gridder. and finnished with wet and dry
the dust is nasty so wear a mask .
good luck
you can only get better with pracatice