Buy a commercial sanding roll. These commercial sanding materials are essentially large rolls of sandpaper. Sandpaper is paper or cloth that has been covered with an abrasive substance. Most commonly the abrasive consists of tiny bits of garnet or emery that have been glued firmly to the backing. Various preparations of aluminum or chromium oxide are also widely used. The abrasive itself is a hard substance that will rub off and wear down the surface to be sanded. The different abrasives are used on different materials. Sanding machines, or belt sanders, usually operate with a commercially-made sanding belt that is designed to fit snugly around the machine's operative mechanism.
Use a commercial version as a guide. Cut open your sanding belt and lay it flat next to a sanding roll. Cut off a piece of sandpaper the same length as the commercial belt. You can also size your belt by pulling out the roll of sandpaper and wrapping it around the machine's mechanism. Make your cuts at a moderate angle. 32 degrees or so would be good. The angles will help you to join the ends more securely. If you are starting a new roll of sandpaper, make an angular cut at the beginning of the roll and then size your piece.
Using a file, or similar hard object, rub off a narrow strip of abrasive at each end of your piece of sandpaper. Each end needs to be thoroughly stripped of abrasive or else the ends will not lay flat when joined together as a sanding belt. The cleaned ends should also be of identical widths. Use a glue that will bond well with your material. Some glues work better with paper or cloth. Rub the glue onto each cleared strip and let it soak in. After it has soaked in apply another layer.
Bring the two cleared ends together. Try not to crease the paper or cloth. The sanding belt should retain its original flexibility. Press the two ends together. Carefully wipe away any excess glue. Place your sanding belt on the side and allow the glue to set. It is important that the glue be completely set before you attempt to use your belt. You do not want it to come apart under the stresses of the machine. Once the glue is dry, take your belt and fit it on the machine's mechanism. It is now ready to use.