A casting is a copy of your “pattern”. Sand is packed around the pattern and carefully removed. A slight draft is necessary on vertical surfaces (sides), like the way “stacking cups” work. Patterns can be made of wood, plastic or metal. Wood patterns should be sealed to prevent moisture being robbed from the sand. Shellac or paint work well.
When making a pattern, increase size 2% for shrink and a % for machining, depending on finish.