Place a cookie cutter in the top left of your paper-lined tray and fill it with 4-5 pieces of each color of rock salt (I like to pick out the biggest pieces). Use a toothpick or skewer to arrange the rock salt pieces into your desired positions.
Add Epsom salts and sifted baking soda into a non-plastic bowl and mix well. Add carrier oil, essential oils, water, and then stir until fully incorporated. Your mixture should be damp but not so soggy that the salt starts to dissolve.
Spoon your salt mixture into your cookie cutter and use your fingers to push it down firmly on top of the rock salt pieces. Keep filling your cookie cutter until you can pack it down level with the rim of the cookie cutter.
Slowly and carefully lift the cookie cutter to reveal a formed salt cake. If you shape starts to sag or melt it means your mixture is too moist. You can add more Epsom salt to the mixture to counteract this.
Repeat steps 1-4 until all of your Epsom salt mixture is gone.
Set your salt cakes aside to dry overnight, then flip them over to dry for another 12 hours.