There are two kinds of gamecube controller mod:
the first was made by dekunukem and it's made with cheap pieces you can get from ebay, and the tutorial to do it is public. However, you need to feed energy to the arduino, and you have to choose what analog (3ds or GC) you're going to use with a jumper (making the whole hardmod permanent).
The second is made by a guy (who I don't remember in this moment) but it uses a special pcb made by him, which you can only get paying for the whole hardmod service.