Uplug power and flip PSU switch off if it has one. Looking at the motherboard power supply either 20 pin or 20+4 pin. Using a wire, connect the green (should only be one on either 20 pin or 20+4 pin power connector) and connect it to the black wire ( there are several wires but any one of them will work).
So you have connect the green (switch) and black (ground). Now plug the power and flip the switch and it should run on itself.
Remember you can only run a PSU with no load for only couple of seconds because it's no good for it. So aviod running the PSU by itself without any components connected to it. But connecting a water cooling pump or hd and medias should be fine while running.
already been answered and the author has already done it chuck...just a bit slow.