1. Get the piece(s) of rubber that you are going to glue. Put the rubber under the same amount of stress and the same condition that it will be under when it is glued. This means that you may need to stretch the rubber so that excessive stretching does not interfere with the adhesive. For example, if you are gluing an inner tube, you would want to inflate it at this point.
2. Wet the surface of the rubber where you intend to glue the rubber. Apply dish soap to the wet rubber surface. With a rag, scrub the surface of the rubber to a lather. Rinse off the soap with a generous amount of water.
3. Dry the surface of the rubber. Blot the area with a paper towel until there is no sign of wetness. Make sure that the surface of the area that you wish to glue is completely dry before applying the adhesive. Gluing rubber to a wet surface will interfere with the adhesive setting properly.
4. Sand the surface of the area that you will glue with the sand paper. Make the surface rough so that the glue has a nice surface to which it can adhere. Do not make an indentation in the rubber; merely scuff the surface.
5. Apply a thin layer of rubber cement to the surface(s) that you wish to glue. Scrape away the excess with the scrap of cardboard. Press the surface(s) into position. Allow the rubber cement to cure according to the instructions on the tube. Force out air bubbles to the edge of the patch by pressing them on one edge.