well the way i do it is, pick an area for the shoot, set camera on tripod etc.
I use a remote switch with a hold function and shoot in 'bulb', or a 30 second shutter speed can work also, then set it on the timer so you have chance to get in front of the lens.
Then, know what you want to paint first...shapes, lines, writing etc.
then basically move around with the light in front of the lens and the light image is captured...but you are not because you are moving and its dark.
This may sound a little crude and other people have probably got better techniques, but this is just how i have done it.
Any help??