If using a bath bomb mold, fill each half of the mold until it’s overflowing, packing in slightly. Press halves together firmly and let mold sit 1 min. Gently tap mold and gently pull mold apart. Let dry 24 hours.
Instead of using an ice cube tray or a mold, try sprinkling the mixture directly into a warm bath. Save any remaining mixture in a resealable container.