You start by recording a data file that maps the location of the dust spots on the sensor. To do this, you need a white piece of paper and a lens that offers a focal length of 50mm or longer. Put the camera in the P, Tv, Av, or M exposure mode, set the lens to manual focusing, and then set the focus distance at infinity. (If you're holding the camera in the horizontal position, turn the lens focusing ring counter-clockwise until it stops.) Next, open Shooting Menu 4, choose Dust Delete Data, and select OK.
Position the paper 8 to 12 inches from the camera, make sure that the paper fills the viewfinder, and then press the shutter button all the way. No picture is taken; the camera just records the Dust Delete Data in its internal memory. When you see the message "Data obtained," select OK. The current date appears on the initial Dust Delete Data screen.
After you create your Dust Delete Data file, the camera attaches the data to every image you shoot. To clean a photo in Digital Photo Professional 4, select the image thumbnail, open the Adjustment menu, and then click Apply Dust Delete Data. The program's instruction manual, available for download from the Canon website, offers additional details.