Sterilize all materials. Boil half the distilled water and use it to thoroughly rinse the measuring teaspoon, the stirring spoon and the inside of your container. Ideally, the container should be new or never have been used to store anything that could be dangerous to the eyes.
Pour the remaining liter of distilled water into your container. Do not allow any contaminants to get into the water.
Pour 1 teaspoon of cooking salt into the water. Cooking salt is better than table salt as it is more likely to be pure sodium chloride. If you have access to pure, chemical-grade sodium chloride, that is the safest ingredient to use.
Stir the solution until the salt is fully dissolved. Once there are no visible salt particles, seal the container. It is now ready to be applied with a sterile eyedropper to an actor's eyes before the scene that requires crying.