The easiest way is to use a regular ATX power supply.
There's a couple of threads on here and tons more elsewhere that show how to power up the Power Supply. Then you just connect the hard drive connector to the M2. ACC and PWR go to the yellow PS wire (+12v) and ground to either black wire. Don't forget, the red wire in ATX supplies is +5v which won't do you much good unless you need USB power.
P.S. Without actually looking it up, I think it's Pin 13 and Pin 14. I know for sure it's the green wire that you have to connect to any of the black wires to jumper it into starting. I've got one here that I used to power my M2 and for some reason, I jumpered another pair of wires. Someone somewhere must have mentioned it. Might just be the sense wire.