Unscrew the retaining screw in the center of the 120v wall socket but leave the faceplate on.
Push the temperature switch into the wall socket, making sure the prongs enter fully. Line up the "pigtail" -- the small metal bar in the temperature switch -- into the screw hole.
Insert the screw through the pigtail and back into the screw hole and tighten it down to secure the temperature switch and faceplate to the wall.
Plug in the grow lights or heat strip and adjust the settings off the temperature switch by turning the dial or knob, depending on the model.