In a large bowl, combine baking soda, citric acid, epsom salt and cornstarch.  Mix well to get any clumps out. Add in essential oil and coconut oil, and give it a good whirl. Slowly add in water, stirring after each addition. It'll fizz a little as you add in the water.

You can divvy this mixture out into ice cube trays, different molds, or use a meat baller to form balls. You could even free form them with your hands, although they might not look as nice.

Allow bath bombs to sit for 24 hours before moving or storing.