Wiring a switch into an existing light circuit adds a great deal of utility to the existing circuit. Now, rather than having to switch the light on and off at the light itself, you'll be able to turn the light off or on at a remote location, such as on the wall by an entryway to the room. Wiring the switch itself will take some simple tools, and some important precautions must be observed.
Turn off the power to the circuit you'll be working on at the breaker box. If you're not certain which breaker switch to use, turn on the light and test the switches one at a time until the light goes out.
Cut the existing power line leading to the light at the position you want the switch to be at, with a set of wire cutters. Remove the outer jacket of the cable for two inches to either side of the cut with your wire cutters. Remove 1/2 inch of insulation from the black and white wires in the cable on both sides of the cut.
Place the bare ends of the white wires together, parallel to each other. Slip a wire nut over the ends of the wires and twist it to the right until it securely holds the wires together. Wrap the base of the wire nut with electrical tape as added insurance.
Loosen the two screws on the right side of the switch with a screwdriver. Place the bare end of the black wire leading to the light under the top screw and tighten the screw to hold the wire in place. Place the end of the other black wire under the other screw and tighten it.
Never work on a live electrical circuit.