I used a Dremel for most of my trimming and shaping with a light grit to clean up after.
Best advice I can give is read a couple of build threads 3 times, measure everything 4 times, and fit test each piece before permanently attaching anything.
Mistakes can be corrected with some effort, but best to make as few as possible with some preparation.
Another great tool is to head to reach out to your local garrison/squad and ask about upcoming armor parties and see if any scouts would bring their kit and discuss there build with them, ask about what they did for various pieces and what they wished they had done differently.