Bring the milk to a boil, then remove from the heat and allow to cool to 115F. In a medium glass or ceramic bowl, dilute the starter with 1/2 cup of the warm milk. Stir in the remaining milk and cover securely with cellophane. Wrap the bowl twice with a heavy cloth and set in a warm draft-free area. Let stand for 4-5 hours. Refrigerate the yogurt before serving.