First you have to become an astronaut.
When you are in space you have to get a full water bag, a tooth brush and toothpaste.
Then put water from your water bag on to your tooth brush.
Next squeeze on some toothpaste onto your tooth brush.
Once you've done that brush your teeth.
After that swallow your tooth paste because you can't spit in space.
Then put away your tooth brush and tooth paste and you're done.