Incorrectly wired three-way light switches have aggravated many a homeowner: You've got a light over the stairwell, supposedly controlled by switches at both the top and bottom of the stairs, but one won't work unless the other one is turned on. It doesn't have to be that way. This wiring plan assumes you are working with new construction or a remodelling project that has opened up the walls, so you can wire the thing from scratch.
Set the electrical boxes in place on the studs, securing them with screws and positioning them a half-inch forward of the stud to allow for drywall.
Run a two-wire cable from the fuse box to the first switch. Do not connect the cable to the fuse box yet. Secure the cable to the back of the box with the brackets provided. Pull out and separate the wires, one black and one white, plus the copper grounding wire.
In the same box, secure one end of the three-wire cable and pull those wires out (black, white, and red, plus the grounding wire). Connect the ends of the two grounding wires to each other with wire nuts. Connect the two white wires to each other so that you're left with a loose black wire coming from the fuse box and loose black and red wires going out to the other switch.
Find the three connector screws on the side of your three-way switch - two on one side and one on the other. Connect the incoming black wire and the outgoing black wire to the two screws that are on the same side of the switch, so one is above the other. Connect the red wire to the single screw on the other side of the switch. Wrap the sides of the switch in electrical tape to seal the connections.
Run the three-wire cable from the first switch to the second switch box and secure it there, separating the black, white, and red wires, plus the grounding wire. Secure the end of a new length of two-wire cable to the second box, separating the wires. Run the other end of that two-wire cable to the light and connect the wires to it.
Back at the second switch box, wire the three-way switch as before: white to white, copper to copper, both black wires on one side of the switch, and the red wire on the other. Wrap the sides of the switch with electrical tape.
Turn on the power at the fuse box and test your switches. Either one should turn the light on and off, regardless of the position of the other one. If it's correct, turn off the power, tuck the wire and switches into the boxes, and secure the switches to the boxes with the screws provided. Once the whole system is sealed, turn the power back on.
Using plastic electrical boxes allows you to connect the grounding wires. If you use a metal box, you'll need to connect the grounding wires to each of the boxes.
Always make sure the power is off before working on any electrical circuit.