Step 1: Set Up
Take a regular shutter speed shot in the light to make sure that your spot is in focus (see picture)
Set your camera's shutter speed (you'll have to set your camera for manual mode) - I use 30 seconds most of the time
Either turn on a time delay or set the shutter speed to compensate for the time it takes to walk 10-15 feet away.
Grab your flashlight and shut the lights out.