I have cracked, scratched and snapped armor in my building and trooping. I have one kit as the shadow stormtrooper so far. There are a few things that you need to go by. As already stated, go slow and don't rush. The faster you go the more mistakes that can happen.
Make sure you measure at least three times and cut once. (Or rather measure twice cut once) If you know the exact measurements you need to go by then the better your kit will fit and the more fun you will have at troops without having to worry about armor bites or needing to get the ca glue or tape.
The number one rule though is to relax. If you are ever unsure about a cut, mark it, take a pic and ask us. We can help.