how exciting! ATA is a pretty simple kit to work with. Having said that, it is possible to mess-up and over-trim. Be sure to read build threads like crazy. The shadow stromtrooper and white stormtrooper are made the same way, so look at every build thread you can if you haven't already. When trimming armor, I've tried a lot of different methods, but my favorite by far is to use a fresh razor blade to score followed my bending and snapping. Nice, clean lines with usually no need to sand.
My understanding is that when red plastic is stressed, it turns white at the stress point. However, heating the area carefully will restore the red color.
Be sure to ask specific questions and post your progress as you go!