how do you trigger the iPhone kill switch?

Some people don't like the idea of a kill switch that can be activated if your phone is stolen. I support the idea for the same reason I support vaccination. If most phones had kill switches, many fewer phones would be stolen. It's like herd immunity. Sure, if it's your phone, it'd be a hassle, but chances are that your stolen phone would not be coming back to you anyway. However, my question is how you go about activating the kill if you do have your iPhone stolen.