Combine baking soda, citric acid and cornstarch in a large mixing bowl. Stir well with a whisk or spoon.
Slowly add 1 tablespoon of carrier oil (like almond oil) while you keep stirring the mixture.
Add a few drops of essential oil (optional).
Divide the mixture over several bowls. Start by adding one drop of blue, red and yellow coloring to each bowl, mix well and then add another. Add a handful of sprinkles to the yellow batch (optional).
Take a handful of mixture in the palm of your hand and squeeze together. The mixture should stick together like damp sand.
Fill up both sides of your mold with the mixture. Layer the colors as you go. Press both sides together.
Let the bath bombs dry overnight before taking them out of their mold.
If you don't like to use sprinkles for your bath bomb, use jojoba beads, edible glitter dust or biodegradable glitter instead.