Simply twisting the appropriate wires together is not sufficient, as they may later come loose. There is likely to be some variation between installations depending on the unit that you are installing and your individual vehicle. These will hold the receiver securely in the dash, fastening it to the inside of the mounting sleeve. Before inserting the receiver into the dash, reconnect the auto's battery and test the system to make certain that all connections are good and the system is completely functional. With the system tested and the gain controls set to your liking, feed the harness and wiring into the dash opening and slide the unit in until it snaps in place.
Some units also come with a trim ring that snaps on around the outside of the face plate.
Be certain to save that tool as well as the old stereo equipment and leftover mounting hardware. You may want to reinstall the old system when you trade or sell the vehicle.