Prerequisites:
The Basics
Overview:
Adding a light switch to control one or more lights is easy.
First, place one or more lights in your map that you'd like to control with a light switch. Don't add a regular light though, add a TriggerLight (available under Light). Also, there are some items under Decoration -> DeusExDecoration that can be controlled by a light switch, such as a CageLight and Lightbulb. Set all of the Events -> Tag values of the lights to the same thing.
You also need to set the Object -> InitialState value to TriggerToggle. That will let you turn the light on and off.
NOTE: You don't need to set the InitialState value for decoration lights like CageLight and Lightbulb.
There are some other properties you can set under TriggerLight, but the defaults are the most realistic setting in most cases. If you put in a ChangeTime, the light will take the number of seconds you put in to fade on and off, sort of like a dimmer switch. However, if you also set bDelayFullOn to True, the light will simply pause for that number of seconds before turning on or off. Not sure how useful that is, but it's there.
Leave bInitiallyOn set to False.
Now add your light switch. The built-in one is available under Decoration -> DeusExDecoration and is called, appropriately enough, LightSwitch. Be sure to put it where the player can easily reach it. About four grid squares off the ground (with the grid set to 16 units) works well.
Finally, set the light switch's Events -> Event value to the same name you assigned to the lights' Tag values in Step 1.
Since you've added some lights, be sure to hit F8 and Rebuild Geometry before running your map.