Sometimes two-way light switches are not just convenient, they're a must. Imagine switching on the light at the bottom of the stairs and having no way to turn it off at the top of the stairs. Two-way light switches in the bedroom also can make pitch-black fumbling toward the bed a thing of the past if you install an additional switch next to your headboard.
Buy two specifically designed two-way switches from a hardware store. You will recognise these because they have three terminals on the back, while common switches have two. The terminals will be labelled "Com" for common, "L1" for Live 1 and "L2" for Live 2.
Buy an appropriate length of wiring cable to run between the two switches. The wire should contain four elements; three sheathed core cables and a bare copper ground wire.
Buy an appropriate length of 14-gauge house wiring cable that contains two sheathed wires and one bare copper ground wire. This will connect a switch to the breaker box.
Replace the old switches with the new two-way switches; prepare the walls for any additional switches.
Prepare the path where new cables will run. There will be new cables running between the two switches, and one running from the second switch to the light fixture. Dig a channel to set the cable back into the wall if necessary.
Switch off the power at the breaker box.
If there is not already power to either light switch then run two-wire house wiring cable between either switch and the breaker box. This cable will be referred to as "cable 1" and the switch as "switch 1".
Connect the black wire from cable 1 to the "Com" terminal at the top of the first switch.
If a light fixture is not fitted you will need to connect one. Use two-wire cable with black and white wires. Connect the white neutral wire from the light cable to the white neutral wire from cable 1 using a cable nut.
Run cable containing three wires (plus a ground wire) between your two switches. You'll see this cable has white, black, red and copper parts. This cable is now known as "Cable 2".
Connect the black wire of cable 2 to the "L1" terminal at the bottom left of the both switches.
Connect the red wire of cable 2 to the "L2" terminals of both switches. Both red and black wires indicate live terminals used in the transfer of power.
Connect the light fitting to switch 2 via the upper "Com" terminal, using the black wire from the light cable.
Connect the earth wires into the ground terminals, switch on the power and test to see if everything works correctly.
Be sure to buy a little more wire than you need for trimming purposes and to accommodate measurement errors.
Some areas might require electrical work to be performed only by licensed electricians. Check local requirements. Always make sure that the wires you are working with are not live. Keep water away from all electrical connections and make sure the surfaces are dry.