This model adapter will work for Blue & White G3s, Yikes G4s, and Sawtooth G4s. For convenience, I will refer to it as the "Sawtooth Adapter". This adapter is the easiest to make as these systems have nearly-ATX-compliant power requirements and wiring schemes.
The modification process for Sawtooth G4s is very easy compared to the other systems. The first step is to remove the -5v line (pin 18) and either discard it or cover it so it can't short out.
Step two is to bring 3.3v to pin 8 on the motherboard and 3.3v to the POK line. There are two ways you can do this, most people choose to solder a 3.3v line to the POK line. The other method relies on knowledge of the motherboard. All nearby similar voltage lines are connected together in the motherboard, so the 3.3v (pins 1, 2, and 11) soldered together. If one line isn't connected, the other two will pick up the slack. You can remove the POK (pin 8) and a 3.3v line from the motherboard connector (pin 1, 2, or 11). Plug the 3.3v line into pin 8, then the POK line into the empty 3.3v spot. The motherboard will then feed 3.3v back into POK and allow the power supply to function normally.
This is how my Sawtooth adapter looks:
For convenience, I also include power adapters to use to connect the Apple case fan to the new ATX power supply. This adapters plugs into a normal hard drive connector and split off connections for another hard drive plus the connection to the fan. The adapter has the ends crimped into the molex connector to give a clean look with no unsightly splices.