You can make this as simple as you want.
Step 1: Parts List
-a 9Volt battery
-a 9Volt battery clip
-a 9 or 12 Volt fan (you can buy these easily from ebay.com, dx,com or a local store)
unfortunately I didn't had a spare fan with plastic around the blades, those are saver and easier to use in this project.
-Wire strippers ( if you don't have this you can use a knife or scissors instead, but be careful)
-a sliding-switch(optional, but really cheap)
-wire cutters and scissors might come in handy!
Step 2: The Making
Then take exposed part of both red wires and twist them together. Like in the picture.
Next you put a little piece of tape over the connection.
Step 3: The Making
Take the other black wire and string it though the left or right pin.
And put a little piece of tape over the connections, make sure the two wires don't touch each other, so insulate them separately.
Step 4: The Making
You can now connect the battery-clip to the battery.
To release stress on the wires, you can tape the battery to the side of the fan.
Enjoy your homemade fan!!!