Combine dry ingredients in a mixing bowl, making sure to break up any chunks.

Add optional essential oil.

Mix until well combined. The mixture should be slightly damp. If it still needs more moisture, or if you're not using essential oil, use a spray bottle to spritz on a little at a time. Don't add too much water!

Press firmly into ice cube tray or mold. You can do a test with just one tablet to make sure it holds its shape before adding more water.

Allow to dry completely, then remove from molds. The time required for drying will depend on the temperature and humidity; it can take from several hours to a day.

Store in an air tight container.