In the event that the case you purchased came with a PSU, then you can skip this step. If not, continue below.
Installation of the PSU is very easy to do. Your case will have a large open slot at the back, usually at the top or bottom of the case. Simply slide in your PSU, making sure the holes for the screws on the PSU line up with the holes provided on the back of the case. The fan on the PSU should be facing out, so that the air will be blown out the case. If your PSU came with two fans, make sure that the second fan is facing into the case. Facing down if it is top mounted and up if it is bottom mounted. Once the PSU is inserted properly, simply secure the PSU with the screws provided.
Now that the PSU has been installed, you can move onto the installation of the motherboard. Before moving on.