To create light graffiti, you or a friend will stand in front of the camera with a light, in a dark setting. After you open the shutter, you can begin moving the light to draw shapes and patterns in the air. If you move quickly enough, the camera won't record a person, only the patterns of light you leave behind. The camera may also record the background if its spotlighted by other light sources, such as the moon or a streetlight.
For best-quality shots, use an ISO setting of 200 or lower; higher ISO settings create more noise, or grain, in low-light images. Use the lowest aperture number on your camera to let in more light, unless, of course, you're shooting in a location that's a bit bright. In that case, a higher aperture number (16 or higher) may darken the scene enough to let you write more graffiti.
Finally, you'll need more time so you can create your graffiti masterpiece, so longer shutter speeds are essential. Experiment with 20 or 40 second exposures, or even 5 minute exposures, to see how the results look. Of course, the longer the exposure, the darker location you'll need.