In a large bowl, combine washing soda, baking soda, citric acid, and salt. Stir well to distribute everything evenly.
Slowly pour in the vinegar while stirring. The mixture will fizz a little (totally normal). Keep mixing until you get a damp, crumbly texture. If you’re using the lemon essential oil, add it now.
You can use silicone molds or trays to mold the pods, but this time I used my 1-tablespoon measuring spoon, which has a round shape. You could also use a cookie scoop, it worked even better than a mold! Just press the mixture firmly into the spoon to shape it, then place the formed pods on parchment paper to dry.
I really wanted them to be super solid, so I lightly sprayed them with alcohol to help harden them even more.
Let them sit out for at least 24 hours to fully harden. If you live in a humid area, you might need up to 48 hours.
Once dry, store them in an airtight glass jar or container to keep moisture out.