Very easy. you'll need: 12cm or 14cm 12v fan 12v pretty much any 12v power supply (the 2$ dollars cheapo will work). 1) Fan: strip off red and black wire, leave the yellow alone. Cut off the plug of your 12v adapter and find the white one (like this http://imgur.com/Ae8nHLl) 2) Wire togheter red from the fan to the white on the power supply, black from fan on black 12v power supply DONE. You may use some grip tape to the fan so it won't move, but it's not necessary http://www.soundproofing-direct.co....pe/acoustaseal3_large_784_general-500x500.jpg OBS: you can wire your 12v to a USB using the red and black cable from the USB cord, but it's going to be very slow, not worth it (5v) OBS:2 you can buy a on/off switch , but I don't feel it's necessary. People will find it more accessible since you don't have to plug off wall every time
If you plan to use a 12v power supply, I'd recommend to use a 120v 2A For some unknown reason my 140mm BeQuiet fan is quieter on the 2A than on 300mA and 500mA