In a mixing bowl, mix together baking soda, salt, corn starch and citric acid.
Next, in a small bowl or small spray bottle, mix water and essential oil. Add liquid to dry mixture slowly. Stir well. If the mixture fizzes a little, it's okay. Be sure to mix the liquid in evenly.
Finally, press mixture into molds. You can use cleaned egg cartons, yogurt containers or just shape by hand. Let sit in molds for 1 hour. Then pop out and let them dry out for 1-2 days before using.
To use: add as many bath bombs as you like to a nice hot bath and enjoy!