Changing a light switch from a two way to a three way switch is not that difficult and adds convenience to any room. You’ll need several things all purchased at any home center or hardware store and you need to know how to use basic hand tools, such as screw driver, plyers, wire strippers. To change a single switch light to three way system (two switches and the light) you’ll need two three way switches, two switch boxes (you can use the existing box if there is one) two lengths of wire (14/3** with ground, and 14/2** with ground), some yellow wire nuts and black electrical tape.
First turn off the power to the light. Assuming that you have an older home, the power for your light is most likely coming to the fixture in the ceiling and then diverted to the wall switch. Remove the light, and check the wiring. If you have a white wire (neutral) from the switch attached to the black wire (hot) of the light then the power is in the ceiling. To make things easier, connect the wires in your ceiling black to black and white to white. This will make the power coming from the old switch.
Locate the position where you want to put you second switch. Cut a hole in the wall. It is not necessary to mount the switch near a stud, (you can buy a plastic box that will mount anyplace, and it has two wings that attach to the back side of the sheetrock or plaster)
You will need to run the 14/3 wire from the old switch box to the new one. (It is easier to remove the box, run the wire, put the wire in the box and reinstall the box) First attach the grounds together with a short piece (6”) ground wire. Put a wire nut (yellow*) and a piece of black tape to prevent the nut from getting loose. Strip a shot piece of the white insulation exposing an inch if bare copper from both white wires, and secure them with a wire nut and tape. Push them into the back of the box out of the way. Attach the bare copper (ground) pigtail to the green screw on the three way switch. Now attach the red wire and the black wire from the 14/3** to the traveler screws on the switch and the lone black wire to the common screw. Be sure screws are tight, but do not over tighten or you will break the plastic sides of the switch. Wrap the switch with a piece of black tape over the screws so they are covered. This prevents incidental contact and prevents the screws from coming loose.
From the hole where the second switch will be run a piece 14/2 to the light. Push the two wires into your new box and install it. (It is easier to put the wires into the box before installing it) Attach the wire the same way you did in the old light switch box.
Attach the black wire from your new switch to the black of the light and white to white. Attach the ground (bare copper) wire to the light fixture bare copper or green screw (if there is one) or to the ceiling box itself with a screw.
Before attaching all the switches and light fixture to their respective boxes, turn on the power and test the light. You will be able to turn the light on and off from two different locations.
*Wire nuts are color coded for different sizes. Yellow wire nuts will handle up to 4 pieces of 12 gauge wire and is what is needed in this application.
**14/3 means 14 gauge 3 wires (red, black and white), 14/2 means with 2 wire (black and white)