Question: how do you whiten your teeth with a banana peel?
Answer: Rub the underside of the banana peel all over your teeth.
Once your teeth have been thoroughly rubbed, let the banana matter sit on your teeth for ten minutes. ... 
Use a dry toothbrush to work the banana into your teeth.
Brush the banana away with a wet toothbrush and some toothpaste, as you normally would.