If you are replacing one switch at a time it should be straight
forward. Turn the light on and then throw the circuit breaker. This way
you can tell it is off because there is no light on. Then just remove
the old wires and place it on the new switch. Here is a pretty good
picture of it and instructions:
http://www.handymanwire.com/articles/3wayswitch.html

or

http://www.electrical-online.com/how...ayswitches.htm

or

http://www.the-home-improvement-web....way-switch.htm

Hope that helps
Pat