You can just use an exacto knife and a metal ruler, go over the area twice, and literally snap it with your hands (or pliers for those tough areas). I use the knife from Rona, can't remember what it's called. For curvy spots, I still use the knife but some swear by lexan curver scissors (never tried). Don't be afraid to mark the line with pencil or from the inside (where no one can see) I even use a marker. You should see the inside of my armour, it's like I drew a grid with permanent markers etc.
Where Wayne is coming from is that tools generally speaking are hard to control especially if you don't have some experience with them. They tend to get away from you and result in a serious mess imho... lots of ABS dust. Bad for your lungs.