4-6 cups water
1/6 cup (~3 tbsp) salt
1/2 cup baking soda
1/2 cup Dr. Bronner's soap

Dissolve water, salt, and soda, boil.
Transfer to a 1 gallon jug. Add soap.
Fill with water (leave room to shake).
Put it in your washing machine, not your mouth.

Source: Melinda Silver via Brian Gray