Wash, stalk and stone the cherries. Weigh 1 kg of cherries and mix with the jam sugar in a large pot. Cover the pot and let the juice stand for 3-4 hours.
Bring to the boil over high heat while stirring. Then boil for at least 3 minutes while stirring continuously. If the jam is still very chunky, mash a little with a potato masher.
Stir in vinegar according to taste. Immediately pour the jam into clean twist-off jars, close them and let them cool upside down.