a dremel is a rotary tool that has a huge number of assorted bits (cut off wheels, sanders, brushes ect). If you do a lot of hands on stuff its a great tool to have.
I upgraded my stock 5.25's in my 95 monte carlo. I talked to some other people that did it and they said it was easy, but the speakers I bought were actually 6.75" so I had alittle more trouble. I was able to get them in using a pair of tinsnips, a dremel, and an angle grinder. I didnt use any fiberglass, but I should have. I dont know if its possible to use the grills that came with the speakers and have it look good. I just stuck with the factory grills.
I'm not trying to scare you or anything, chances are your install will go better than mine, but problems do come up.