You can do this in any order but this is how I do it.
1.Pull mirror triangle trim panel. This is just held in by two clips on the back side. You can pull on the top of it and then the bottom.
2. Pop off the door lock cover. The piece that moves back and forth when you lock the doors. There will be 1 7mm bolt behind there.
3. Remove 1 7mm bolt hiding behind the pull handle/arm rest. Just bend over and look and you will see it hiding in there.
4. Pull the trim piece around the door handle. You can use a screw driver but be careful not to mark up your door panel. Use a putty knife and your fingers if you have one.
5. Now the door panel is free to move. Slide the door panel up about an 1" by lifting straight up on it. Now you will be able to pull the door panel towards you. Do not try to unplug the light in the bottom of the door unless you cant get it out. I always use a pry tool and pop the light out of the door panel from behind. You will see it just have 4 bumps in the light that push through the door panel to hold it in. If you try to unplug it you will end up with the light bulb falling into the light itself and it can be a pain to get out.
6. Now you just need to unplug the switches. I always roll the window down before I unplug it. This way you can make sure your speaker is not to deep and will hit the glass.
This will be much easier if you have a second person to hold the panel while you unplug everything. Now that the panel is off you will see that GM likes to use crazy speaker plates that there speakers are attached to. Technically GM uses a 63/4" speaker. Yes you bought the correct size because no one really makes a 63/4" speaker. So do yourself a favor and get the adapter plates.
http://metraonline.com/part/82-3002 These will pop right in the front door and should only cost you about $15.00. Otherwise you will be trying to fit your speaker in the a hole that is to big and end up having a gap all around your speaker and just getting one/two screw sideways to hold your speaker in. This is no good...
Another thing you will want to do...If your truck has the tweeters in the front door leave them unplugged when you put everything back together. Your new speakers have tweeters on them. It will just make is sound bad by having more tweeters hooked up. Reverse to put the door back on.
Rear doors come off pretty much the same way. You wont need the adapter plates for the rear doors. So just get 1 set of the rings they come 2 per bag. I have the rings at the shop if you cant find them I can get them to you.
If you run into anything let me know and Ill help if I can....