Article: Ensure that the circuit for the room you are working in has been flipped off. This will prevent accidental electrocution and reduce the risk of fire.  Most circuit breakers are located in the garage or basement, but placement will vary from home to home. When you find the breaker box, locate the breaker that controls the lights for the room you are working on. Flip it to the off position to keep power from traveling down those wires. Most electrical panels label switches for the different rooms in your home.  Use a voltage tester to check that power is no longer flowing to that room. This will affect the way that you perform the installation. You can tell if the power is coming into the switch by removing the light switch panel. If two black wires are coming into the switch box, then the power is coming into the switch first. If there is only one black wire, then the power is coming into the switch from the light fixture. You can usually use an electrical tester to be sure. A 3-way switch will not have the words on or off written on it. Before you start installing, take a look at your new 3-way switch to identify all of the terminals that you will be connecting wires to.  Traveler wire terminals - These are located on each side of the switch towards the top of the switch.  Ground wire terminal - Older switches may not have this, but all new switches must. This is usually a green screw located on the top or bottom of the switch, mounted to the frame.   Common wire screw - This is located on the left side of the switch, and is a different color than the two traveler terminals.

What is a summary?
Switch off the circuit. Determine if the power goes to the light or the light switch. Replace any regular switches with the 3-way switches.