More in Electrical
Shut off the power at the main circuit panel (Image 1).
Use an electrical tester to be sure that the power is off at the fixture.
Disconnect the wires to the old single-pole switch (Image 2).
Disconnect the wiring cables from the box.
If the old box is attached to a wall stud with nails or screws, use a pry bar or screwdriver to remove it.
Pull the existing switch wiring cables, and the wiring cable (or cables) for the fixture controlled by the new switch, into the box through separate rear access holes (Image 1). Secure the cables to the box with clamps.
Use a barrel clamp to connect the bare or green ground wires from both incoming cables.
Replace the wires to the old switch just as they were attached before.
Attach the incoming fixture wires to the new switch. This connection will depend on how the power is routed to the fixture controlled by the switch.
Install the double switch box cover (Image 2).
Restore power to both circuits and test the operation of both switches.