Things you will need:
* digital camera capable of long exposures or, preferably, a bulb setting
* light source (i.e. flashlight)
* dark area (a room, a park far from the city)
* remote shutter release for your camera (not strictly necessary, but makes life easier)
Set up your camera on its tripod, and make sure it's on bulb and set to your remote. Get your friend to stand approximately where they will be painting, and make sure you're properly focused.
Right before you start painting, open the shutter using your remote (you can do this without a remote by just pressing the shutter release, but there might be some blurriness due to the camera shaking). If there is any ambient light at all (like there was in our pictures) you'll need to paint fairly quickly, or have a smaller aperture (these photos were all f2.8). When you're done painting, close the shutter.
Here are our efforts, done at Iona Beach after an aborted attempt at photographing stars. It's still a work in progress.
Now here it is, done right:
credit: The Areographers
credit: Atton Conrad