Mix together citric acid, baking soda, cornstarch, and oil in a bowl
If you choose to use an essential oil put in 8-10 drops.
Split this mixture into smaller bowls to make different colors.
Add in a few drops of food coloring and mix together.
Put on rubber gloves and rub the mixture into the food coloring between your fingers to get a consistent color. Just as if you were rubbing sand between them at the beach.
The mixture should feel like damp sand.
Press the mixture into the mold of your choice and pack it in tight.
Let dry for 24 hours
Drop into a bath and enjoy!