Natural Cure For Canker Sores 1. Place 1/8 of a teaspoon (about 0.5ml) in a cup. 2. Add one drop of water. 3. Stir into a paste 4. dab a little of the paste on the ulcer and leave it there for about a minute, or two. 5. rinse the area with completely (your mouth may feel somewhat parched). 6. Usually one application will do the trick, but sometimes, if the ulcer is severe, you may need to reapply the next day. Either way, for me, the ulcer disappears within 24-hours.