Here is how it’s done:
Set your camera on the tripod and take a sample shot with flash / lights on. This will help you verify that your composition is OK.
Set the exposure to a relatively long value. Stop down the aperture as much as you need. If you are outside do nothing. If you are inside – this is the time to close the lights.
Make the click. Once the shutter is open use your flashlight to light the stuff that you want to “paint”. You can use the flashlight as a brash, and “smear” the light, just like you would have done with brush and paper. Or, you can use the light as a pen, and do precise work. Areas where you go slowly will be more lit then others. Be careful not to linger to much over the same stop – you will burn it. (The machos amongst you will correctly identify this as the “I forgot the iron on the shirt” phenomena).