Step 3: Storage
Hydrogen peroxide is UV sensitive, so be sure to store your toothpaste out of sunlight, or in an opaque container.
I thought using a pastry bag would be cute and convenient (since I have them on hand). The only drawback to this is that there's no way to cap it! So I made the toothpaste extra paste-y, and problem solved. Using a tube squeezer from a paint shop helps keep the end closed and gets out every last drop.
You can also find plastic tubes to fill at outdoor and camping stores, like REI (which I might do next time, if I can think of a cute label to put on it).
Other options include storing your toothpaste in a squeeze bottle or jar.
Of course, if all of this sounds like too much effort, you can always just dip your toothbrush in hydrogen peroxide, and then in baking soda. Job done.