Measure out your ingredients. You will need 3 cups of baking soda, 1 1/2 cups of citric acid, 2 cups of cornstarch, 3 cc of the fragrance oil of your choice and .15 cc of powdered water-soluble dye. If you do not want to have a scented bath, do not add the fragrance oil. Powdered water-soluble dyes come in a variety of colors.
Combine the ingredients in a mixing bowl and stir with a spoon.
Measure out portions of about 1/4 cup to1/2 cup of the mixture.
Pour the mixture into separate sealable bags to store or give as gifts.
Turn on the water to fill your bathtub.
Pour 1/4 cup to 1/2 cup of the mixture into the bath while the water is running to allow the mixture to dissolve into the water and release the color.