This is the first step, where you take the controller apart with a screw driver and cut a few plastic separators off if there are not necessary. In this picture we have cut the cross-hair top left rubber off so it provides more room for the USB extension. Also I have cut the plastic part next to the wire exit hole so I can fit the wire in easily!!
Step 2: Step 2 - the back panel
This is the back panel off the NES controller, just chop the top middle and the top right screw guidence plastic so the USB flash drive can fit easily.
Step 3: Step 3 - modifing the curcuit board
Eventhought this sounds difficult, it simply isn't. All that is needed is just get the area which the 'female' part of the USB extention lead fits with the flash drive, and cut! make sure that the screw holes are still there so you can screw it back together and the button features are there also, this is only so the buttons are pressable. [BEWARE, THE CURCUIT BOARD IS VERY BRITTLE, TRUST ME - I HAVE BROKEN A FEW!!]
Step 4: Step 4 - Adding the flash drive and extention lead
This is the most crucial part of the whole procedure. Screw the casing off the flash drive and connect it to the female part of the USB lead. Next you need to place it carefully into the NES casing, and making sure that all of the unnecessary plastic poles and features are gone so you can easily fit in the flash drive. Some modification (i.e cutting) is required to the NES so it can fit well. After that curve the extension lead around the separators and place through the lead hole at the top right of the NES case.