You want to start out with a really thick and hearty foundation of foam alphabet puzzle pieces for this recipe, and then thoroughly moisten them. If the entire bathroom isn't soaked, then you have not added enough water, and if you skimp on the water, your rocks will absorb too much toothpaste and become chewy.
When your letters are completely wetted down, add a few rocks to the sink, and then season with toothpaste to taste. Some people prefer a more subtle hand with the toothpaste, but I like to really pile it on.
Stir all ingredients vigorously enough to splash water onto the mirror and anyone standing near you.