Place the tin in a saucepan and fill with water halfway up the sides of the tin. Bring to the boil over high heat. Cover, reduce heat to medium and cook for 3 hours. Check every so often to ensure that the pan isn't about to run dry, as the tin can explode if there's no water in the pan. Top up with water as needed.
After three hours, remove the tin using tongs and place in an ice bath. Let cool before opening.