How to make a long exposure of a SLR camera produce the effect of a picture?

Leave the shutter open long enough for the camera's sensor to "see" the movement of the subject.

Long-exposure photography is often used for night photography to produce a different look than during daytime hours. Create a sharper photo by extending the camera's shutter opening time to absorb more light. It is usually fixed with a corner frame to avoid blurring. And enlarge the aperture to avoid underexposure, causing the photo to be completely dark.

With this technique, the scene remains fairly dark while the photographer or assistant takes a light source—perhaps a small flashlight—and moves it to draw the pattern. The light source can be turned off between strokes. Objects that are usually stationary in the scene are depicted by a brief turn on of the studio lights, one or more flashes, or a gradually expanding aperture.

Extended information:

Long exposure photos are usually not taken at standard shutter speeds, but in BULB mode. B mode gives us the freedom to choose exposures lasting longer than 60 seconds. Another healthy practice when using long exposures is to turn on the ISO noise reduction setting, as even at low ISO sensitivities, the amount of light hitting the sensor can cause noise in the final image.

This is due to a phenomenon known as "hot pixels" where certain pixels of the camera sensor literally get very hot and create noise. When our image is zoomed in at 100%, this noise is seen as red/green/blue pixels in our image.