Press the capture button, run to wherever your camera is focused on, and turn on your light source.
Draw the picture you want, trying not to shine the light source directly at the camera. This will produce a "twinkle" where you shined it from (see picture).
If you want to stop a line segment and start a new one, you can cover the light source with your hand and move it over to where the new line segment will start. If you have a flashlight, just turn it off, move it over, and turn it back on! <-- Much easier!
When you're done drawing:
If it's a simple picture (a square, circle etc) you can go over it again
If not, don't bother trying to go over it. It will probably get messed up.
DO NOT TURN THE LIGHTS BACK ON UNTIL YOU ARE SURE THE CAMERA HAS FINISHED TAKING THE PICTURE!