Mix ¼ cup water and oil together in a small bowl. Place the baking soda in a separate medium bowl, and stir in the water mixture. You want a dry, thick paste. If the mixture seems too dry, add in a little more water, up to a total of 1/3 cup.
Fill the silicone molds with the paste, packing the baking soda mixture down tightly. Let dry completely (this might take a couple of days, depending on the humidity level).
Carefully remove the shower bombs from the molds, and store in an air-tight container for up to 2 weeks.
To use, place the Peppermint Shower Bomb in the corner of your shower, and take a hot shower with it there in the corner. It will break down as the water hits it, and release the lovely scent while you shower.