I just did this ...3 days ago :) It was with a usb controller to a set of buttons to play stepmania... Anyhow - open up the controller. Each button will have 2 wires. It's much easier to run 2 wires for each button instead of trying to use the same (sometimes present) matrixing inside the controller. Find the pads for every controller button, and solder a wire to each side - then connect it to the switches. I like the use of stripped cat5 cable twists.