I wouldn't reccommend a Dremel any way, doesn't drill through plastic rather melts its way through. I have a pair of pin vises, which will accomadate 1/8th thru #80 drill bits and hand drill the holes I need in plastic.
You figure out what size hole you need, select the size bit to use (sometimes a cheap dial caliper helps) and drill the hole. If you are drilling into a rounded plastic surface, start by spinning an exacto knife with a #11 blade at the exact location you want the hole. This will keep the drill bit from slipping away from where you really want it. I would reccommend that, if you have one available, use an old model to practice on.