Fill a glass or bottle, such as a clean and empty commercial mouthwash bottle, about two-thirds of the way full with filtered, distilled or mineral water.
Add food-grade, 3-percent hydrogen peroxide to the water, making a mixture of about two-thirds water and one-third peroxide. No matter how much mouthwash you make -- a single-use dosage in a glass or a whole bottle's worth -- stick to a ratio of one part peroxide and two parts water.
Add a few pinches of baking soda to the solution -- roughly a pinch for every capful of mouthwash -- and gently stir until the baking soda dissolves.
Rinse your mouth for about 30 to 60 seconds, swishing vigorously without swallowing, and then spit out the mixture.