Hang flowers upside down in a window until dried. I chopped them up a bit with a knife to make it easier to disperse in the bath bomb.
In a large bowl, measure and mix the baking soda, citric acid, and corn starch.
Melt the coconut oil (not to make it hot, but just enough to make it a liquid) in the microwave. Add 12 drops of lavender essential oil to the coconut oil.
Using a whisk or your hands, gently add in the oil mixture to the baking soda mixture, a tablespoon at a time. The mixture should have the consistency of damp sand - just enough to make a form when you squeeze it without it crumbling. If you find that your bath bomb is still crumbling, many people recommend adding a spritz of water or witch hazel.
Stir in the rose petals.
Using a bath bomb form, fill and press one half of the sphere mold so that it mounds a bit higher than the brim. Fill and press the mix in the other half to even with the brim.
Using a twisting motion, press and squeeze any excess out of the sides of the bath bomb until both halves are tightly together. Gently tap one side with your finger tips to release the mixture. Slowly twist the top in an upward motion to take off. You can let your bath bombs dry in the bottom half if you choose. Dry for at least 24 hours before use.