In one sentence, describe what the following article is about:

You can use a glass or plastic bottle as long as it is dark-colored; light will cause the hydrogen peroxide to degrade faster. Make sure that you are using distilled or filtered water. Use only 3% hydrogen peroxide. Anything higher could be damaging to your oral and dental health. Store the bottle in a cool, dark place until you are ready to use it. Pour some mouthwash into a cup. Swish and gargle it in your mouth for about 30 seconds, then spit it out. Rinse your mouth afterwards with water, then discard any mouthwash left in the cup.

Summary:
Pour 1 cup (240 milliliters) of lukewarm water into a dark bottle. Add 1 cup (240 milliliters) of 3% hydrogen peroxide. Close the bottle, then shake it to combine the ingredients. Use the mouthwash no more than twice daily.