I heard its tricky, but how to I do it?

First person got close with the answer. Turn of the G5, then hold down the ‘T’ during boot to start it up in “Target disk” mode. NOW, plug them together with a Firewire cable. The G5 will act like an external drive. Just copy all your stuff over.

Another choice it to plug them together directly with an ethernet cable. Turn on file sharing on the G5.