1. Place the baking soda, citric acid, and cornstarch in a large bowl. Use a pastry cutter or large whisk to remove lumps and mix well.
2. Add the water SLOWLY, 1 tbsp at a time. You have to be very careful because if you add too much water too soon, the whole thing will fizz up. Just be patient and keep testing by grabbing a handful of the mixture and squeezing it in your hand. As soon as it feels like damp sand (that would be perfect for making a sandcastle), you've added enough water.
3. Add the essential oils, and after every few drops, use your pastry cutter or a fork to mash them into the mixture.
4. Place about 2 tbsp of mixture into each dome, and use the back of a spoon or ice cream scoop to press down and smooth.
5. Leave overnight.
6. Carefully remove from molds and package in a pretty Mason jar or clear gift bag with ribbons and decorations etc.
7. To Store: They will store for up to 6 months in a cool, dark cabinet.
To Use: Take into the shower with you, wet it under the stream of water to get it going. Place the steamer at the far end of the bath/shower stall. You want it to last, and if it's under the stream of water it will disappear too quickly.
Caution: Because you are bringing oils into the shower, make sure you use a shower mat, and wipe out the shower floor when you are done.