How do I adjust the belt so that it doesn’t slip to the side?

The first thing you will need to do is find out the size of the belt sander. This is the length of the area that goes around where you will be putting new sand paper. The size should be in the directions. Having the proper size sanding belt keeps the belt from moving so much. Some belt sanders have a small screw to adjust the tension (the screw's may be Phillips head, flat head, or possibly an Allen wrench). Again, please refer to the owners manual to make sure both for the location and the manufacturer's directions. Other's may have a manual knob to turn. Caution: Do not overtighten!