The hardest part of installing a second wall switch for an existing light fixture, when the power source connects to the existing switch, is pulling new wires through the wall.
Step 1: Use the breaker or fuse in the circuit panel to turn off the power to the existing switch and light fixture.
Step 2: Remove the cover plate and detach the mounting screws holding the switch in place. Gently pull the switch out of the box to expose the wires behind. Use a voltage sensor to double check that there is no power running through any of the wires.
Step 3: Aside from the green or bare copper ground wire and the two wires attached to the switch, there should be a pair of wires in the box connected together with a wire nut and electrical tape. This wire pair is the telltale sign that the power source connects to the switch.
Step 4: Install an electrical box for the new switch and run a red, a black and a white wire from the new box to the old switch. Mark the white wire on either end with a black piece of electrical tape.
Step 5: At the new switch box, attach the red wire to the gold screw on the paired side of the new switch. Attach the black wire to the silver screw. Attach the white wire to the gold screw on the opposite side of the switch. Attach the ground wire to the green screw.
Step 6: Replace the old switch with a three-way switch by attaching the black wire running up to the fixture to the silver screw on the paired side of the switch. Attach the ground wire to the green screw.
Step 7: Use a wire nut and electrical tape to attach the black “hot” wire coming into the box to the black wire running to the new switch.
Step 8: Use a wire nut and electrical tape to join the grounding wire from the new switch to the other grounding wires.
Step 9: Attach the red wire running from the new switch to the gold screw on the paired side of the replacement switch.
Step 10: Attach the marked white wire from the new switch to the gold screw on the non-paired side of the replacement switch. Turn the power on and test the switches before securing everything back into place.