I found this recipe for bath fizzies in the book Martha Stewart Crafts.
I was going to order molds online such as something similar to the ones here. But then when in the Dollar Tree they had the silcone ice cube trays. I have seen them at various times of the year too in different shapes. And I think silicone would be easier to get the fizzies out.
1 3/4 cup Baking soda
2 cups Cornstarch
1 cup Citric acid
Sift 1 3/4 cup baking soda, 1 cup citric acid, and 2 cups cornstarch through a sieve to remove chunks. Fill small spritzer bottles with water and add about 6 drops of food coloring. If you want different tints, use multiple spritzer bottles with different food colorings.
Pour 1 cup of powdered mixture into a glass bowl. Lightly spritz, stirring after each spritz, until powder is desired color. Add spritz water slowly, so mixture does not fizz. If mixing two tints, alternate colors as you spritz. Check the consistency of powder with your fingers; when it can be tightly packed or shaped, stop spritzing (this may take a little while).
Select an essential oil. Add 5 drops if it's one of the stronger scents (peppermint, lavender), 6 if it's a weaker one (lemon, grapefruit). Mix well. Firmly pack mixture into small baking molds. Allow mixture to set for 2 hours, then pop out carefully. Repeat with different tints for remaining powder.
You will have to experiment how much is good for one bath. The silicon ice cube trays that I used - 2 were good for one bath. I packaged them in snack size ziplocks and did a cardboard topper on it that I stapled on the bag with directions on the back.