Add water to baking soda a little at at time (up to 1 cup) until a thick paste forms. Add more baking soda if the mix is too wet. It will dry faster with less water!
Add up to 30 drops of essential oils and mix well.
Spoon the paste into the mold of your choice and level it out. It settles down to a flatter surface on its own. I use silicone mini doughnut pans or muffin pans. Silicone is the easiest to work with but you can also use metal pans with liners, or try coating the forms with coconut oil first. This one would be great to use.
Let air dry for 2-3 days until fully dry and hard.
You can also add the essential oils after the tablets dry. In this case, you can dry them out in the oven. Bake the tablets at 350° F for 15-25 minutes, depending on the tablet size. Then add 1-2 drops of oil, per tablet, just before your shower. (This method is good if you want to be able to use whatever kind of oil you feel like on a given day!)
When tablets are completely dried out, store in a sealed glass or plastic container.
To use: If the tablets don't already contain oils, add 1-2 drops of the oil you plan to use onto the tablet and then place in your shower. When you're ready for your shower, place 2 of them on the side or toward the back where they will get wet but not be in the direct stream. They will last for one to two showers.