P.S. This is my first instructable, so please pardon the garbled text and blurry images.
Step 1: Parts Needed (Or at least the one's I used)
Here's a list of what I used:
1. Fan out of dead power supply (120mm I believe)
2. 6v Lantern Battery
3. Soldering Iron and Solder
4. Hot Glue and Gun
5. Diagonal Cutting Pliers
6. Toggle(On/Off) switch from lantern that the 6v battery came from.
7. Wire Strippers
Step 2: Taking Fan Apart...Or Not.
1. Flip fan over and snip the little bars holding the actual motor and blades in place (2nd Pic). There should be around 4 little plastic bars to snip.
2. The fan should look like this when you're done snipping (picture 3)
Step 3: Installing the Switch...Or Not
1. There's probably a little pin connector on the end of your fan leads, so just go ahead and snip that off.
2. Strip the end of the wires.
3. Pick a line to snip. I chose the positive lead. Strip ends off original lead, and the piece you snipped off in the last step.
4. Choose a switch and line it up.
5. Solder that bad boy.