In a small bowl, combine the baking soda and salt or Epsom salt, and mix well until combined.
Spray or drop small amounts of water into the baking soda mixture and stir until it is the consistency of slightly wet sand. The mixture should hold together without crumbling when squeezed, but should not appear wet at all. You may need to add slightly more water if it hasn't achieved this consistency yet.
Add the essential oils and powdered menthol crystals and stir to combine.
Quickly, press the mixture firmly into silicone molds, greased muffin tins, or any other greased container.
Set aside and let sit at least 24 hours (48 is better) or until hardened.
When dry, remove and store in an airtight container until ready to use.
To Use:
Place 1-2 melts just outside of the shower stream on the floor of the shower.
Let the hot water and steam do its work, and enjoy!
If the melts dissolve too fast you may have added too much water in step 2. Stop adding water as soon as the mixture holds together without crumbling, but doesn't appear wet.
If the melts dissolve too slowly, pack them a little more lightly on the next batch, allowing more space for the water to get in and dissolve the melt.
Store away from moisture and light. Use within 1 month for the strongest scent and best results.
Notes
For the powdered menthol crystals, I simply bought crystals and crushed them between parchment paper to get a powder.