Step 1: What you will need
1.Computer Power Supply Unit
2.Soldering Iron + Solder
4.dc adapter power jack 2.1mm
Step 2: The diagram
Of the other voltages available, the +3.3V connection delivers ample current, it's just not a very useful voltage. The +5VSB (5 volts, always on), -12V and -5V are normally very low current lines and are of little use to us.
The green wire, pin 14, is the on/off switching line. To turn the power supply on, the green line needs to be shorted to a Common line. An easy way to do this is to insert a jumper between pin 14 and pin 13.
Most power supplies require a load across one or more of the outputs to operate. The link I give above shows how to add a resistor across the 5 volt side of the supply to act as a load.
The smaller connectors coming out of the power supply use the same color codes. As an example, a connector with a yellow, a red and two black wires will have +12 volts (yellow), +5 volts (red) and two commons.
To use the power supply, for 12 volts, you'd connect a yellow wire to the + input of your project and a black wire to the - input. For 5 volts, you connect a red wire to the + and a black wire to the -.