If you look at the big (20 or 24 pin) connector from the power supply, you'll see one green wire. Short that green wire to any one of the black ground wires & the power supply will turn on. Yellow is 12V, red is 5V, and orange is 3v.
I disagree to a point. It depends on the power supply. i have done a number of these as well and it all depends on the model of PS. I have ones where i could simply jump the wires. Then i have had others where it required a "load" for it to stay running for any period of time. I solved the issue easily by using a LED push button:Just run the Jumper from 14 to 15, 16 or 17 and the PS will work, no extra wires needed. I dont use any resistors. I believe you need a resistor if there will not be a load on the PS, if your using a wiper motor, that becomes the load.