Clean the rubber that needs to be repaired with a solvent-based cleaner. Work in a well-ventilated area, and wear gloves, safety glasses and a respirator mask. Solvent cleaners can be dangerous to your health, so all safety precautions should be carried out. Use a soft cloth to apply the cleaner. Allow the cleaner to dry before proceeding.
Cut out the area of the rubber that needs patching. Use scissors or a utility knife to make the cut. Make sure all edges of the rubber are smooth and even. If the area that needs repair is large, you can save the rubber to use as a patch.
Sand the edges of the hole with rough grit sandpaper to allow the silicone RTV to stick better to the rubber surface.
Squeeze a line of silicone RTV over the rubber area that needs patching. Smooth out the silicone with a small paintbrush. Feather the edges of the silicone out over the original surface of the rubber. Place the rubber patch over the silicone if you have it. Wipe away excess silicone from the area with a damp cloth.
Allow the adhesive to dry for two hours. Apply another coat of silicone if necessary, to raise the level of the patch to the level of the rest of the rubber. This is only necessary if you have to patch a deep hole. Allow the silicone to dry for 24 hours before using the rubber item.