Locate the hot wire feeding the switch and disconnect it from the switch. Run an electrical wire down the wall to new box. Install a 12-3 NM wire that has a ground wire. Leave about 6 - 8 inches of wire hanging out of the boxes.
By Timothy Thiele, About.com Guide
Locate the hot wire feeding the switch and disconnect it from the switch. Run an electrical wire down the wall to new box. Install a 12-3 NM wire that has a ground wire. Leave about 6 - 8 inches of wire hanging out of the boxes.