There are different ways to sharpen a knife with sandpaper, mostly depending on your personal preference. Before you start sharpening the knife, clean it of any dirt or particles. One way of sharpening a knife with sandpaper is to cover a brick or block of wood with sandpaper and run the knife (lying flat) across it in a circular motion, making sure to spend equal time on each side of the knife. It will take a few minutes for each side.
Another way to sharpen a knife with sandpaper is to keep the knife at about a 22-degree angle and stroke it across the sandpaper, making sure that the tip also gets sharpened. To get an approximate 22-degree angle, put the knife edge against the sandpaper at a 90-degree angle, then a 45-degree angle, then half that. Don't use heavy pressure when sharpening the knife, and move the knife away from you as you draw it over the sandpaper.