Figure out how much soap base you will need. Some soap molds explicitly note how many ounces of soap they hold, while others don’t. If yours don’t, try this trick: fill up your soap mold with water and then carefully pour the water in your mold into a large measuring glass or cup. This should tell you roughly how many ounces of soap base you will need to fill your mold. For my molds, I needed at least 10 ounces of soap base