In a mixing bowl, combine 1/4 cup of salt and 1/2 cup of washing soda.
If you’re using them, add about 20 drops of your favorite essential oils. This step is totally optional, but it does give your laundry that fresh, natural scent.
Grate an entire bar of Castile soap. Once grated, add it to the mixing bowl.
Slowly add vinegar, pour in 1/4 cup at a time, mixing as you go. The reaction between the washing soda and vinegar will make a thick, paste-like consistency. I used about 1/2 cup total. Be careful with the amount of vinegar, If the mixture gets too wet, the pods won’t harden enough and will break in your hands.
Scoop the mixture into your ice cube trays, pressing firmly to ensure they hold together.
Let them dry for at least 24 hours. If you want to speed up the process, placing them in the sun helps.