Having worked with and laid a lot of fiberglass, the best way to work with it is to sand it. It'll take a while, and wear a mask!, but that's the only way to work with it without cracking it (plus it'll stay smooth without any extra work).
Get yourself a dremel tool with a sanding drum, turn it on high, and you can cook through a lot of 'glass rather quick.
If you're trying to actually cut a piece of fiberglass (so that you can use both pieces when you're done), get a dremel tool with a couple cutting wheels. Get yourself glasses or goggles because when those cutting wheels shatter, they fly all over and very fast (been stabbed in the forehead a few times).