Installing a Power Supply:
1. Make sure New Power Supply is compatible with all other components in your computer, making sure also that it provides sufficient wattage throughout your Computer.
2. Make sure and check that dimensions of the power supply will fit in your case or tower properly before installing. (Not all Power Supplies are the Same in dimension).
3. When ready to install make sure old power supply is unplugged from any wall Outlet, or any other hardware device within your tower or case.
4. After taking all the screws out and removing the old power supply, carefully slide the new power supply into the old power supplies housing.
5. Replace the screws securing the power supply into place.
6. After power supply is secure start hooking up its connectors to other hardware devices starting with the main connection to the motherboard.
7. After all components are connected plug your power supply into a wall outlet and try booting up your computer.
8. If you have any issues with boot up try going and resetting your jumper settings for your CMOS/Bios.
9. Any other issues may result from a bad power supply replacement (contact vendor or store you purchased power supply from and check on warranty).
Just something to remind people most stores will not take returns on Electronics Devices that have been opened, so when purchasing an electronic component make sure to find out if there is any warranties, and any reasons for voiding receipt.
Try to avoid forcing any connections into other hardware this may cause electrical failure or power management loss.