Attach the Dimmer and Finish Installation
There will be three wires: one black, one white, and a separate ground wire that may be bare copper or sometimes wrapped in green. Identify the three different colored wires and attach the dimmer with the wires in the same manner. Connect the black wire to the black wire, etc. Use pliers to gently twist the ends of the wires together and then secure them together with a wire nut until it is tight and won't turn anymore.
Carefully push the wires back into the electrical box.
Screw the new dimmer switch to the electrical box and then replace the cover plate.