I use a old fan power cable
and an old usb female connector from old usb extender cable
The socket usb has 5v and ground pins on the side, central pins are for data
(be careful, You must watch pinout, usb wire color is not mandatory)
We have to be careful because fun cable take 12v from power supply and we need 5v
We must open the connector and connect it to 5v red cable of power supply instead of yellow
You must disassemble the connector
and change pinout 
It is the final cable
To start the raspberry you may need to jumpstart atx power supply
Francesco Amirante 