gamecube modding

can anyone tell me exactly how to do it, if it's possible?

There's different ways :
If you could get sd media launcher then get gcos to play games without (hard) modding or
XenoGC (modchip) was a popular one my friend has it works great.If you're also looking to play roms (nes snes n64 etc...) i advise that you should get sd media launcher then get the apps (emulators) also you would have space on the sd card to put apps and roms on.hope this helps