Pick up the hard disk drive cable and notice the connector on the end of the cable. There is a tab on one side of the connector, as shown below.
tab
Pull the cable over toward the socket and fold it to align the connector with the socket. Make sure the side of the connector with the tab lines up with the side of the socket with the notch. Then line up the holes over all of the pins and push the connector onto the pins.
Caution
If you do not correctly align the holes with the pins, you could severely damage your computer when you push in the connector.