Wash and stem grapes. Crush in a saucepan and add just enough boiling water to cover.
Simmer slowly until skins are soft, about 10 minutes, then strain through a damp jelly bag or double layers of cheesecloth.
Refrigerate juice 24 to 48 hours. Carefully pour off clear liquid without disturbing sediment; filter again if desired.
Sweeten to taste if desired and heat until the juice begins to boil.
Fill jars immediately, leaving 1/4-inch headspace. Process pints/quarts 5/10/15 minutes and half-gallons 10/15/20 minutes at 0–1,000/1,001–6,000/above 6,000 ft.