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.
Step 4: Glue It All Up Baby!
1. Take ends of leads from fan and stick them to the terminals to see if there's any juice from your battery and test your switch.
2. Wrap leads around spring terminals and put a little dab of solder on them for maximum awesomeness.
3. Take back of fan motor and hot glue that sucker on the side of your battery, making sure that the wires are coming off the side of the battery.
4. Glue your switch in place. (I chose the top of the battery.
5. Glue your wires down and away from the fan blades.
6. Place near soldering station to prevent cancer and other horrible diseases! haha
Wasn't that easy!? (And probably painstakingly over-descriptive) It's my first instrucatable...so what do you expect? =)