Unpack the PC case and place it right way up on the work surface (if it does not have a PSU already fitted, unpack your separate PSU and place it carefully on the work surface). Find the mains cable for the PSU, but do not plug it in just yet.
Check your case instructions to determine how to remove the outer casing (or in some styles, side cover panel). This should give you clear access to the inside such that you can fit all your new equipment. Once this is done, carefully lay the case on its side, so that the opening faces up.
You will notice at the rear of the case there will be a large hole (at the top in most cases). You will need to install the PSU, such that the fan outlet and power socket on the PSU will face outward and the power connectors all hang loosely inside the case.
The example on the right demonstrates a correctly fitted PSU. Notice the four fixing screws (circled in red) that hold the PSU in place. It is also important to make sure that any 'ventilation openings' on the PSU case itself, face toward the inside of the PC case. The PSU has a secondary function: to draw hot air out of the PC case and push it out of the back.