1. Prepare the Dry Ingredients:
In a large mixing bowl, sift together the bicarbonate soda, citric acid, corn starch, and Epsom salt. Sifting helps remove any lumps and ensures an even mixture. Whisk the dry ingredients until well combined.

2. Add the Oil and Essential Oils:
In a separate small bowl, mix the carrier oil of your choice with the essential oils. Slowly drizzle the oil mixture into the dry ingredients, stirring continuously with a whisk or spatula to distribute the oils evenly.

3. Add Color (Optional):
If you're using colorant, add a few drops to the mixture. If you're using liquid color, be cautious not to add too much liquid to prevent the mixture from fizzing prematurely. Mix well until the color is evenly distributed.

4. Moistening the Mixture:
Fill a spray bottle with water. Lightly spray the mixture with water while stirring constantly. You want just enough moisture for the mixture to hold its shape when pressed together. It should feel like damp sand. Be careful not to add too much water, or the bath bombs may start to fizz.

5. Mould the Bath Bombs:
Once the mixture holds its shape, pack it tightly into your bath bomb moulds. Press the mixture firmly into each half of the mould and then press the two halves together if you're using round moulds. If you're using silicone moulds, pack the mixture into each mould until it’s firm.

6. Let the Bath Bombs Dry:
Carefully remove the bath bombs from the moulds and place them on a tray lined with parchment paper. Allow them to dry and harden for 24-48 hours in a dry, cool place. Avoid touching them too much during this time to prevent cracking.

7. Storage:
Once completely dry, store the bath bombs in an airtight container or wrap them individually in plastic wrap to prevent moisture from getting to them.