Combine the baking soda & water together with a spoon. Fill a silicone muffin liner or pan halfway with the mixture. Add a few drops of each of the essential oils, and then fill the rest of the way with the baking soda mixture. Bake at 350 for 20 minutes. Once cooled, add a few more generous drops of each essential oil to both the top & the bottom of each disc. Or, for maximum scent power - store the discs as is, and add the essential oils to both sides shortly before using. Store the discs in an airtight container or plastic bag for up to 6 months.