First note where the old power supply cables were plugged in. Do the same with the new power supply. The connectors will either fit or not. There is the 20 or 24-pin main connector for the motherboard, then usually another 4-pin or 8-pin connector for the motherboard. Then there are connectors for the optical drive(s) and the hard disk drive(s), and maybe one or more 6-pin connector(s) for the video card (if you have one). That should about cover it.