I actually followed the Garf instructions for all the rock in my 37. I didn't use salt, but just dry sand etc, but think salt would definately be an improvement as my rock is very heavy.... A "mold" is just basically using dry sand, crushed coral, or salt(probably best) to make a pattern for the bottoms, sides, and caves. IE to make an arch, lay a mound of dry salt, and then put the cement mixture over it. To make a cave, use a base of the mixture, then put mounds of salt where you want the caves, and then cover with cement. I also used rubber glove "balloons and baggies, but the caves look less natural. The boxes come into play for multi-branching tonga rocks.