Weigh out 250 grams of baking soda.
Zero scale and add 125 grams citric acid.
Zero scale and add 125 grams cornstarch.
Zero scale and add 125 grams epsom salt.
Combine well, crushing any lumps.
Spray in witch hazel, alternating with mixing, just until mixture holds it shaped when squeezed tightly in your hand. Be very careful not to add too much witch hazel at one time before stirring.
Add essential oils. I like 12-15 drops per recipe which equals about 4 drops per bath bomb.
Loosely pack each side of a bath bomb mold, heaping a little extra on each side. Squeeze the 2 sides of the bath bomb mold together. At this point you can either leave the bath bombs in the mold until they're dry or take them out to dry. Just experiment and see which way you like best. If the bath bomb sticks in the mold, tap it with the edge of a knife to help it release from the mold.