There's a good chance the 24 pins of the socket on the PSU match up in the same order with the 24 pins of the the motherboard socket.
A cable mod should always be tested with a voltage meter before ever connecting it to computer stuff.
You can sort out the wiring with a voltage meter. Find the +5Vstandby line on the 24-pin cable by connecting the red lead of a voltage meter to each pin while the black lead of the meter is grounded to the PSU case (any bare metal, but do not stick anything into the PSU because of the exposed high voltage there). Next, find the Power-On line by using a 1/2 watt resistor rated anywhere from about 200 ohms to 500 ohms. Connect one end of the resistor to case ground, the other end to each of the other 23 pins, until you find the one that turns on the PSU fan. The reason to use a resistor instead of a plain piece of wire is to limit the current to a harmless level when (not if) you ground the wrong pin. Once you turn on the PSU this way, you can sort out all the other pins to find their voltages, but there's a good chance the Power-Good line will measure slightly different from the wires for the +5V rail. Also the Power-Good line will probably dip more in voltage when grounded through that 200-500 ohm resistor than the +5V rail wires will.