it's super easy. for the front doors, you open the little cover in the door handle and remove that screw. then you remove the screw in the pocket of the panel where the window switch is in. then in the top outside corner (the upper right hand side on the right door) and along the bottom of the panel there are two screws. take those out, disconnect the wiring blocks for the door locks and windows, and then you can pull the door panel off. the whole panel comes off and it's easiest to start from the top where it meets the window. once you have it off, you can remove the factory speakers and install your own. your best bet for doing that is to get some wiring harnesses that just plug into the factory wiring and give you the two coaxial speaker connectors. also with the front doors, you CAN install a crossover (to take out the bass) so you can run the factory tweeters along with the speakers, but if the speakers you put in have them already, it's too much high frequency sound for your ears. after you put the speakers in, make sure you test them before you put the door back together (and save yourself some work if you messed up). when you put the panel back on, make sure the plastic standoffs are in each slot in the door panel and then just line them up and puch them into the door. they will click as you push them in. push all around the perimeter of the door so you push them all in as much as possible. as for the back doors, you take them off the same way, except they don't have a screw on the bottom of the panel and you don't have to disconnect a wiring panel for the power locks button. obviously if your speakers require some extra installation or something special, do it.
whew! that's a long one. i went ahead and put in a panasonic dvd-player 7" touch screen, two sets of polk audio db570s, and a little 100w bazooka sub. enough to let me know it's there without rattling the car.