Once the PSU is installed, route the 24-pin power connector and supplemental/CPU power connector through your case and plug them into the motherboard.
Once you put the power supply in the case, it’s generally as simple as screwing it into place with four screws at the back of the case. Or with some cases, you'll need to re-install a bracket (see above, right) that you'll first need to remove in order to slide the power supply in, then screw the four mounting screws into that. Once the PSU is installed, route the 24-pin power connector and supplemental/CPU power connector through your case and plug them into the motherboard. The 24-pin connector is usually somewhere near the center-right of the motherboard, while the PSU power connector plugs in at the top left corner.