Im assuming your talking about the front. The rears are the same kinda. Its not all that hard actually, just follow these steps:
1) remove armrest. (2 screws)
2) Remove the doorhandle trim. (1 screw, hold the hande out as you do this)
3) remove the window crank (this is a bitch, there is a little metal retaining clip. use a really small hex key and fish around under the handle, or opposite the handle. itll be there. be careful when you get it though, it pops with some force!)
4) Pull of the door panel. ( start at the bottom and start pulling, it will just pop off. After there are no more pops left lift the panel up and toward the rear of the car. Jiggle it a bit to get it off.)
5) By now you shoudl see you plastic sheeting held on by grey stuff. Gently pull the plastic off so you can reuse it. It helps if you leave it attached on one side.
6) the speaker can come out 2 if need be. Just 2 screws.
TIP- that grey stuff stains clothes!!! (dont ask how i know!)
To put it all back together just do everythign in reverse order.
i ve done it myself and if ur doin it in garage or fairly clean area , u can ditch the hex nut idea and just use a towel or t shirt by holding the towel taught wiht both hands like a sheet of paper and working it behind the window handle. keep in mind the clip will wind up in canada so keep an eye out for it , IT WILL FLY
I used a pair of needlenose pliers to get in there, grab the clip and pull it out. It was a bit tricky to see what was going on in there but I didn't have any problems and the clip didn't go flying off into space.
To get the crank handle off, spend 3 or 4 dollars for a clip removal tool, they are primarily made for GM handles but it also fits nissan perfectly (as well as my 55 Plymouth). You can usually find one at any auto parts store, check around the "Lisle" specialty tools and the "HELP" small parts section. Also the opposite end of the tool has a crook that is great for loosening door panel push pins. Besides, we always need an excuse to add another tool to the toolchest.