Step 1: Strip SNES controller cable
Alternatively (what I did), you could replace the SNES cable with one from something else, that way you don't need to chop up the controller. In my case I had a broken NES controller that I borrowed a cord from. It is also recommended that instead of cutting the cable you get a SNES controller extension cable and cut that up. In either case the steps are the same.
Step 2: Powering the controller
To draw power from the parallel port attach a diode to pins 4-9 on the parallel port connector according to the diagram below. The +5V end attaches to the white wire on the controller (make shore that the diodes are aligned correctly, the black stripe should point away from the parallel port).
Otherwise you can strip a spare USB cable and attach the controller's power (white) wire to the red 5V wire on the USB cable and the ground (brown) wire on the controller to the black/ground wire in the USB cable.