Based on the industrial process of vacuum forming, a rectangular piece of plastic was clamped in a holder and heated over a metal plate. When the plastic was soft, the holder was swung to the other side, over a mold of the object to be formed. Then, pressing a handle on the side of the unit, it created a vacuum, sucking the plastic down over the mold and shaping the plastic to it. When the plastic cooled it solidified, making an impression of the item.
Various molds came with the kit, but almost any small object could be used as a mold.
Because very hot surfaces were easily accessible to a child (or adult) playing with the toy, it almost certainly would not be sold today.