Layout your proposed switch leg on paper and present it along with a materials-to-be-used list to the Authority Having Jurisdiction (AHJ), the municipal Building Permits Department, when applying for the wiring permit. This is a simple project, but you do need to pull a permit and have the requisite inspection because you are running new wiring. The National Electrical Code (NEC) and local electrical codes require it.
Mount the light switch 46 inches above the finished floor. Use the electronic stud finder to make sure that you won't be drilling or cutting into a stud when cutting the opening for the "Old Work" device box.
Use the "Old Work" device box as a template to outline the cutout. Drill a 3/8-inch hole in each corner of the cutout. Remove the cutout using the portable jigsaw. Cut on the inside of the line because you want as tight a fit as possible.
Turn off the circuit breaker controlling the lighting circuit on which you will be working.
Lower the ceiling light fixture from the lighting outlet box. The fixture may be held in place by two or more #8-32 screws or by a center hex nut. Disconnect the circuit wires.
Run the 12/3 cable from the lighting outlet box to the wall cutout for the switch device box. The easiest way to do this is to work from the attic or attic crawl space. Drill a 5/8 inch hole down through the wall's top plate, the horizontal framing member, into the wall space where the switch cutout is located. In the attic, you can run the cable along the sides of the floor joists or perpendicular to them through holes drilled through them. Drill all holes so that the nearest edge of the hole is at least 1 1/4 inches from the nearest edge of the floor joist. When running the cable along the sides of joists, position it so that it's at least 1 1/4 inches from the nearest edge of the joist. When running it along the floor joists, secure it within 12 inches of the lighting outlet box and at intervals not exceed 4 1/2 feet with staples. These are all NEC requirements.
Insert the ends of the cable into the device box and lighting outlet box far enough for it to extend 6 to 8 inches from the box. Push the device box into the wall cutout and secure in place by tightening the mounting wings screws.
Remove the cable outer jacket using the razor knife. Be careful to not damage the insulation on the individual wires.
Strip 3/4 inches of insulation from the ends of the insulated wires. Re-identify the white wire as a hot wire by taping the full length of its insulation with black plastic electrical tape. Re-identifying the white wire in this way is an NEC requirement.
Make loops in the ends of the wires in the device box. Attach the black wire and the re-identified white wire to the brass screws on the switch. Place the looped ends under the screws in a clockwise direction and tighten the screws securely down on them. Attach the bare copper grounding wire to the green screw on the switch strap. Don't put the switch in the box at this time.
Splice the black wire from the circuit breaker panel to the black wire of the switch loop. Hold the stripped ends of these conductors side by side and twist tightly together in a right-hand twist with the Lineman's pliers. Complete the splice by screwing on a wire nut.
Cap off the re-identified white wire and the white wire in the lighting outlet box with wire nuts. Don't install the lighting fixture at this point. Call the AHJ for the required inspection.
Pass the inspection. Secure the switch in the device box using the two "captured" #6-32 screws. Install the switch cover plate.
Mount the new light fixture. Splice the black fixture wire to the re-identifies white wire, the white fixture wire to the white wire, and the bare ground wire to the ground wire. Secure the fixture to box. Turn on the circuit breaker.