Wash grapes. Remove stems and discard. Press pulp from the skins with fingers. Boil skins 20 minutes in just enough water to prevent sticking. Cook pulp until soft, then press through a colander to remove seeds. Add skins to pulp. Measure. Add 3/4 cup sugar for each cup of grape. Boil to the jellying point. To test this, dip a large spoon into boiling mixture. Tilt spoon until mixture runs over the side. When it reaches the jellying point, it will stop flowing from the spoon in a stream and divide into two distinct drops which run together and leave the edge of the spoon in one large sheet. Stop the cooking at this point. Pour into hot jars and seal.