i litterally did this this evening, my brothers xbox one brick died, and im a pc guy, so i just gave the xbox 12v and 5v, and it was golden.

just put a switch on the green wire on the 24pin motherboard wire to any ground, and the power supply will turn on.

the brick has the required voltages labled clearly, and ignore the power on (usually blue) and the rsense wires in the xbox power brick.