A plaster cast of your hands could be the perfect detail when it's time to decorate for Halloween. METHOD: Combine the alginate powder with water according to the instructions on the package. Pour the mixture into a bowl and submerge your hand. When the mixture is dry, carefully remove your hand. Combine the plaster according to the instructions on the package, pour it into the mould and let dry completely before loosening the mould. Making a hand mould requires approx. 120 g Alginate (depending on the size of the container and hand).