I'll work on a diagram as well, but for now I guess I'll explain it through text.. This is for a 901:
You are going to need the following:
1 - Dead battery (or live if you are that anxious hahaha)
1 - switch (I de-soldered the open/close button off of a broken cdrom drive)
1 - Soldering iron
1 - Tube or reel of solder
1 - pair of scissors
1 - wire stripper (or something to cut and strip wires, I have used wire cutters and scissors to do it as well)
1 - pair of needle nose pliers
1 - roll of electrical tape
1 - 4-6" of heat shrink tubing (optional)
Ok, so.. The brass fitting at the end of the battery is actually 2 pieces. The inner piece is what supplies the power, and the outer supplies the ground. They are separated by a gasket. What you need to do, is hollow out the bettery and then somehow press the brass fitting out of the other end. I just put a screwdriver inside the battery, held the shell, and hit it against the ground a couple times. It popped right out.
Once the brass fitting is free from the shell, take a good look at it. You will notice that there is a place on the outer rim that there are 2 indentations in the brass. Tehre may even be part of a wire already soldered there. This is where you will be soldering your ground wire. Remember not to use too much solder, and to keep it all on the inside. If it spills over, you won't be able to get it back into your battery shell.
For the positive lead, you should see some solder on the inner fitting right next to the hole. Solder your positive wire, keeping sure to keep a good distance from the outer ring, as not to touch the positive to the ground.
Now, get your USB cable, and strip off about a 4" section. Keep the outer tubing as you will be needing it later. Cut off the green and white wires, as you will not be needing them. Now trim the black and red wires down to about 1.5-2" and strip the ends. Get your switch, and solder the red wire from the usb onto one of the 2 prongs.
Now here is what I did to somewhat keep things as safe as possible. I took the wire tubing, and slid it over the positive wire that I soldered to the brass fitting, as snug against the fitting as possible.. Then I taped the ground wire to the tube with electrical tape, to hold everything in place. I did that to make sure that they were as separate as possible.
Now slide the wires through your battery shell, and press the fitting back into it. The wires should be stripped, and sticking out about 1.5-2" Now you just solder the positive lead to the other prong on your switch, and solder the 2 ground leads together. Electrical tape or heat-shrink your exposed connections, and find somewhere suitable to mount your switch. I mounted mine on the usb cable itself. that way, I would get my heat shrink around the tube. If you are going to mount your switch on the battery itself, make sure to put some electrical tape on it first, to avoid touching your positive leads to the casing.
Voila! You should be done! Test it out, carefully, and it should be a fully-functional USB power source! Congrats!
I'm begining to understand. 
