Wiring a second light switch to control one light can be helpful in a large room or on a staircase. When you wire two switches to control one light, the set-up is called a three-way switch. The power comes from the breaker box to the first switch. You then wire the second switch to the first switch and both switches to the light. There are a few different ways this can be accomplished, but this seems to be the easiest way with the least amount of electrical work required.
Turn off power to the room you will be working in. Remove the screws holding the light switches face plate to the wall. Remove the screws securing the light switch to the electrical box. Pull the light switch out.
Cut a 14-gauge red traveller wire to the length needed to run the wire from the first switch to the second switch. Strip one inch off both ends of the wire. Use a pliers to bend the wire into a "U" shape.
Attach one end of the red wire to the free screw on the first light switch. Tighten the screw to hold the wire in place. Run the red wire over to the second switch. Attach that end to a screw on the second switch and tighten it into place.
Locate the black wire running from the first switch to the light. Loosen the screw on the switch holding this wire in place and free the wire from the switch. Re-run the wire from the light to the second switch.
Cut the wire at the second switch. Strip both ends of the wire and bend them. Place one end around one screw on the switch and the other end around another screw. Tighten each screw to hold the wire in place.
Re-run the black wire from the second switch back to the first switch. Re-attach the bent end of the black wire around the screw and tighten.
Locate the green wire running from the first switch to the light. Re-run this wire to go to the second switch. Unscrew the wire nut at the first switch if you need to. Cut the green wire at the second switch. Strip both ends. Cut a 4-inch long piece of 14-gauge green wire and strip both ends.
Place the two cut ends and one of the short piece's ends together and secure them with a wire nut. Bend the short pieces free end and wrap it around the grounding screw in the electrical box.
Run the green wire from the second switch, back to the first switch if you needed to unscrew it from the wire nut to rerun. Screw the ends of all three green wires back together. Restore power.
Use a circuit tester before performing any electrical work to make sure that there is no power being supplied to the switches you will be working with.