All you need is the harness and antenna adapter and dash kit. All are available at your local wal mart... just read the packages and look for your car. For the stereo, you undo the panel under the steering wheel, then pop off the dash trim (be sure to disconnect any wires behind there... mine is a low-option car, so all I have is the fog light switch)... then you unscrew the stock stereo, and plug the new one in. This is assuming you've already connected the harness to the aftermarket head unit previously, which is much easier. It helps getting the dash trim off if you put your car in 1st and put on the e-brake... for me anyway. As for the 6x9s, the bottom of the seat just pops up. You will then see bolts along the bottom (I *think* 4, but not sure off of the top of my head)... undo those, and the seat back raises upwards and off. Might take a little work, but you'll get it. You can then take the rear deck cover off, and viola, 6x9 access. Keep the black plastic adapters... the aftermarket speakers mount back into them. If anything is wrong here, someone please correct me, as I haven't messed with my dash in a good while lol.
PS: If you EVER think you MIGHT run aftermarket amplifiers, I'd highly recommend running RCA cables and a remote wire while you're behind the dash. Power/ground wires can easily be done later on.