For many night shots, a wide aperture with a small f/number and a high ISO setting are preferable, so you can freeze any motion with a fast shutter speed. When painting with light, the opposite applies because movement is the key factor you want to capture.
Select the Manual (M) shooting mode on your camera and use a low ISO of around 100 to 200. Dial in a narrow aperture of around f/11 and set the shutter speed somewhere between 10 and 30 seconds, depending on how much time you need to create your light painting.
You'll also need to set the focus distance manually, so select MF mode. Pre-focus on the area in which you're going to be using the lamp to create the painting. On many cameras you can also enable Manual Focus Peaking (MF Peaking), a visual aid to show which parts of the image are in sharpest focus.
A good creative tip is to manually select your white balance setting. For example, using Daylight, Cloudy, Shade or Tungsten settings will effectively alter the colour temperature of the light, giving a progressively warmer or cooler effect. If it's available in your camera, shoot in RAW quality mode which enables you to change the white balance setting when processing the image in editing software such as Canon Digital Photo Professional.
Start simple by using the camera settings that we've suggested, and use a torch or the light from your smartphone to create basic shapes or maybe even write your name in light. You might find that you need to use a shorter or longer exposure, or a wider or narrower aperture, to get the effect that you want. Experiment with settings and you'll soon get a feel for what works best.