Turn off the circuit breaker supplying power to the receptacle. Insert the blade of a noncontact voltage tester into the receptacles slots to ensure there is no power. If there is power, the tester lights up and/or chirps. Do not proceed until you are sure there is no electricity to the receptacle.
Remove the cover plate screws with a screwdriver. Most cover plates are held in with countersunk flat-head screws. Remove the cover plate and set it aside.
Remove the two screws securing the receptacle to the electrical box with a screwdriver. The screws are located at the top and bottom of the receptacle. Pull the receptacle out of the box. Be careful not to kink or break the wires behind the receptacle.
Guide the electrical box extender over the receptacle and install it into the old box. The flanges of the extender should sit on top of the new finished surface.
Push the receptacle back into the electrical box, sandwiching the extender between the receptacle and the finished surface. Be careful to not kink or break the wires.
Install the new, longer screws (supplied with the extender) through the top and bottom flanges of the receptacle, through the extender, and into the threaded holes in the original box. Tighten the screws with a screwdriver.
Reinstall the cover plate and the original screws with a screwdriver.
Turn on the circuit breaker, and use the voltage tester to ensure the receptacle has power.