Add water to the milk boiler by unscrewing the whistle. Fill the water in the boiler. Do not overfill the boiler, overfill will result in water spilling out of the whistle while boiling milk. Screw in the whistle to the milk boiler. Keep the boiler on a stove and pour milk in it. Do not heat the boiler without any milk. Water is heated first and then the heat is transferred from the water to the milk. The double boiler design limits the overflow of milk. This ensures that the milk is heated slowly and thoroughly without the risk of burning.