hey dude how's it going? This is a pretty common question. Do a search on it in the forums and you should get all the info you could ever want plus more. To start you off with, cutting into the parcel shelf itself will make your car unroadworthy, however, the stock 6" speakers in the back are screwed into a piece of metal that is spot welded on. This spot welded piece of metal can be removed with a chisel or something similar without making your car unroadworthy as it is not structural.
The next issue is fitting them in. The method i'd suggest is simply mounting them to the parcel shelf itself (you'll notice the stock speaker GRILLS are mounted to the parcel shelf itself but not the speakers themselves). So, a few holes drilled in the parcel shelf and you can mount them on there. Thing is, they won't fit in sideways as you'd probably imagine them put in because of the 'dip' in the parcel shelf. But if you mount them long ways (i.e. the 9" part of the 6 x 9's runs as though it is coming from the front of your car going to the rear of the car) they will fit.
Ok, that's a start. Do a search on removing the parcel shelf or getting access to the rear speakers as well if your not too sure on that. You'll pretty much need to remove the back seats along with a few bolts and bits. The seat can be tricky the first time round, but once you get the hang of it it's easy. If you get really stuck and can't find any more info then let us know and i'll see what else i can post up to help. cheers, Bryce.