When running new wiring pay attention to the amp rating for the circuit you are extending. For a 15 amp circuit use 14 awg (american wire gauge) cable, for 20 amps use 12 awg, 30 amp: 10 awg, 40 amps: 8 awg, 50 to 60 amps: 6 awg. Most lighting and receptacle circuits will be 15 amps and make use of 14/2 or 14/3, but some of these circuits may also be 20 amps, so be sure of the rating and use the proper cable and devices for your project. Depending on the gauge and box depth, there is a limit to the number of wires allowed inside an outlet box. If adding new ones would force exceeding the limit it may be possible to add a junction box to the circuit.
The best way to remove the insulation without nicking the copper is to use a wire stripper. Most strippers have a set of cutters for the most common gauges. Gently twist the strippers back and forth once or twice, and pull the insulation straight off the end.
Insert a clamp, with wires attached, into the junction box hole and from inside the box, push the included nut over the wires and onto the clamp threads. Turn the nut as far as possible onto the threads and then use a screwdriver and hammer to tap it down until it’s tight.
Use a straight edge held flat against the top of the base cap to keep your line straight as you pull the knife through the caulk several times until it’s cut cleanly.
Split the sheathing on the cable leaving about 1/4in still covering the wires. Cut the sheathing to remove it and strip off about 3/4 inch of the plastic insulation from the end of each wire. Avoid nicking or scratching the copper wire while removing the insulation.
Try to remove the wallboard cleanly so it can be reattached when you’re done. The baseboard can be reinstalled without replacing the wallboard, but this will create easy access for pest such as rodents, as well as drafts from cold air during the winter.
Cut drywall patches to fit the space on each side of the new outlet box and attach them to the 2x4 support with drywall screws. Tape the seams with mess tape and cover the whole thing with joint compound and smooth it out. Apply several coats to build up a smooth finish that can be sanded and painted to touch up.
Hold the fixture up to splice the wires. Usually the first wire splice will aid in supporting a small light, but have a helper assist with supporting a large, heavy fixture while making the connections. While still supporting the weight, attach the base to the mounting strap with the screws and tighten the base to the ceiling.