Summarize the following:
Turn off the computer then open the case and remove the hard drive. The enclosure is most likely secured with Philips head screws at each corner. Removing them is easily done with a Philips head screwdriver. The hard drive should slide onto the circuit board. Male connectors protrude outwards and should align with female connectors that they slide into. They should slide together with minimal effort. Be careful not to force the connection as the connectors can be fragile. If you are experiencing issues pushing the connectors together, check to make sure the connectors are aligned correctly. Once the hard drive and circuit board are connected, place the unit back into the enclosure. Replace and re-tighten the screws that hold the enclosure together. To use the external drive, simply plug it into the appropriately labeled/sized port on the computer. Depending on the size of the hard drive you may have dual cables, one to connect to the computer and another to connect to a power supply.
Remove the internal hard drive from the computer. Open the enclosure. Place the hard drive onto the circuit board. Line up the male-female (opposite) power and data connectors. Push the male-female connectors together. Place the unit back into the enclosure. Close the enclosure. Plug the external drive into your computer.