Is this hard? / Will it damage my stuff?
This will not modify your controller in any way, and the wiring is extremely simple, so if you do everything carefully there should be no risk to your controller, and you can unhook it at any time to use with a N64 console.
What you will need:
Arduino - $30
Processing 1.0 - free
Arduino Software - free
3 pieces of wire - free (I hope)
Step 1: Wiring the controller
The controller only uses three leads: +3.3V, signal, and ground.
Looking directly at the plug, ground is farthest left, signal is in the middle, and +3.3V is on the right. Using the wire, connect ground and +3.3V to the respective pins on the Arduino, and connect the signal lead to the Digital 2 pin on the Ardunio.
NOTE: If you have other code on your Arduino, you should disconnect the controller and upload the new code from the next page to the Arduino before powering it up with the controller attached.
Connect the Arduino
Connect the USB cable and the Arduino will have power.