Assuming that you are using a fairly modern ATX PC power supply, it's simple:

Join 2 black wires together. This is your negative supply (0v)
Join 2 yellow wires together. This is your positve supply (12v)

Join the green wire to a black wire. This switches the power supply on.

If you are using a decent quality supply, this is all you need.

If the supply shuts down, fit a car tail light bulb between a red wire wire and a black wire. This puts a load on the 5v circuit, and helps to stabilise the supply.

I'm running 2 chargers from a slimline 250watt PSU this way, with no problems.