Fully inflate your mattress. Use a pump for a large mattress, but avoid filling it with too much air.
Spray the mattress with a soap and water solution. Fill the bottle three-quarters of the way and then add a couple squirts of dish soap. Look for areas that begin to bubble. Leaks often occur around the seam, so check there first.
Circle each leak with a pen or marker.
Fully deflate your mattress on a flat surface. Clean the damaged area to prevent debris from getting under the patch.
Cut a vinyl patch. It should be a quarter inch bigger than the rip or tear on all sides. Round the corners so that there are no edges that can be pulled loose. For a temporary fix, use electrical tape.
Apply adhesive to the area around the tear and to the back side of the vinyl patch. Smooth the patch onto the leak and make sure no bubbles are beneath it. Leave this to dry for 24 hours.