The idea is trigger the "down" action on the joystick by releasing pressure on a special butt sensor. To connect the butt sensor to the joystick I just used a simple mono audio cable. In order to wire it into the joystick, I drilled a hole in the side of it and added a basic audio jack.
Take a look and see how the "buttons" underneath the joystick map to the actual board.
Solder one long wire to that wire (it's blue on my joystick). Solder another long wire to the ground (black on mine).
I wanted my butt pad to plug into the joystick through a mono cable so I soldered the two wires to the mono jack I installed earlier.