Assuming you would like to know how to connect a PSU (power supply unit) without a computer, here are the steps:
First, identify which end of the PSU powers the motherboard. This is typically denoted by a 20 or 24-pin connector. If your PSU only has one large connector, it is most likely an older model and will not work with newer computers.
Once you have found the correct connector, locate an unused power cable coming from the PSU and insert it into the matching socket on the motherboard. If there are no other cables plugged into the motherboard, you can plug the power cable in now.
Now that the PSU is providing power to the motherboard, you can go ahead and turn on the switch on back of the unit. You should see some LEDs light up indicating that it is receiving power.
If everything is properly connected, you should now be able to boot up your computer as usual.