From David Fisher,
Your Guide to Candle & Soap Making.
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There are many different options for molding your bath bombs. The key is to get the mixture packed really really tightly.

The most popular mold seems to be ball-shaped - I'm using a clear, plastic, two-piece Christmas ornament here. Overfill each half with the mixture, and press them together firmly. You want the edges of the mold to just barely come together (or be slightly apart) while getting a really good pack on the mixture inside.

Let the molded ball sit for a minute or so. Then tap the side of the ball firmly with a tablespoon. The side of the mold should gently lift off. Repeat on the other side.

Even with your mixture perfectly balanced, you'll occasionally get a ball or tablet that will crumble or be flawed/broken. No problem. Just crush it up, put it back into the bowl and remold it. If your bombs are coming out consistently too crumbly - add a tiny tiny bit more water. That will help the mix to stay together better.