Using hydrogen peroxide as mouthwash is simple: just swish some 1% to 3% hydrogen peroxide around in your mouth, then spit it out. Like you'd do with any kind of mouthwash, right?
Why 1% to 3%? Some people feel that 3% is a bit strong to use as a mouthwash. You can try 3%, or water it down with some water. (Some 3% hydrogen peroxide with an equal amount of water added will give you 1.5% hydrogen peroxide.)
Using hydrogen peroxide as mouthwash will kill bacteria and viruses in your mouth. However, it takes time for hydrogen peroxide to work, so this is not going to remove all the bacteria and viruses in your mouth – it will just lessen them.
Using hydrogen peroxide as mouthwash can taste a bit weird. Also, any time hydrogen peroxide is killing germs, it will foam -- so there can be some foaming when you are using hydrogen peroxide as a mouthwash.
If you want to get at the bacteria in your gums, try it this way: brush your teeth, then floss between your teeth, and then swish with peroxide. This way any gunk is out of the way, and the peroxide can get in to the gums more easily. Swishing for a minute or longer will be more effective than a shorter swish.
Another idea for getting peroxide in to your gums: Put hydrogen peroxide into a Water Pik and then spray the peroxide at your gums, as you usually would do with a Water Pik. I think the people I've heard of doing this have used a mixture of half water and half 3% peroxide. (The mixture is 1.5% peroxide.) Three percent hydrogen peroxide might be too strong to spray into your gums.
You can also try using hydrogen peroxide to brush your teeth - some people use a mixture of peroxide and baking soda.
If that doesn’t appeal to you, you might like to soak your toothbrush in some 3% hydrogen peroxide. That will kill the bacteria and viruses in the bristles of your toothbrush, which will also, in a round-about way, make for fewer bacteria going into your mouth. (Ick.)
Gargling with hydrogen peroxide will get the hydrogen peroxide in contact with the back of you mouth and throat. The longer you gargle, the more time it has to kill germs. I don’t think this will prevent all illness, but it seems logical that it’s a good idea to do this when colds and flus are around. Gargling with peroxide won’t kill all the germs in your throat, it will just lessen them. Peroxide takes time to work. But use some care, as peroxide can also be harmful if used to long or too often.
Gardening: attach sprayer to garden hose; use to dilute peroxide and spray on the garden
Spray bottles for peroxide