To bleed a cast iron radiator, you will need a radiator key and a cloth. First, locate the bleed valve on the radiator. It is usually a small valve located near the top of the radiator. Insert the radiator key into the valve and turn it counterclockwise until you hear a hissing sound. This will release any trapped air in the radiator. Hold the cloth under the valve to catch any drips, and close the valve when a steady stream of water is released.