If needed, add 1 cup water to grapes in a large stockpot. Bring grapes to a boil, crushing the fruit with a potato masher to release juice & pulp until the fruit is very soft.
Measure the juice/pulp and add 3/4 cups of sugar for each cup of grape juice measured. Bring juice to a hard boil that can't be stirred down and reaching jellying point - about 10-15 minutes
(Jellying point is 8 degrees above boiling point of water, or 220 degrees here in NE Texas, but your gel point could be slightly different based on your altitude.
If desired this grape jelly can be water-bath canned. Sanitize canning jars and heat lids. Pour hot jelly into sanitized 1/2-pint canning jar leaving 1/4" headspace.
Wipe any drips from rim of jar and top with 2-part canning lids.
Then remove sealed jars from canner and allow jars to stand in a draft-free place undisturbed for 24 hours before removing the bands.
NOTE: Quantity of finished jars of Grape Jelly depends upon the amount of grapes you harvest & juice you extract. Just add 3/4 cup of sugar for each cup of pulp/juice extracted.
NOTE: It’s best to add as little extra water as possible. If necessary add 1 cup water when simmering grapes to give enough moisture until the grapes begin releasing their own juice. But if you start with previously-frozen grapes you can just thaw them in the refrigerator overnight and they’ve already given up enough juice to simmer the whole grapes with no added water necessary.
NOTE: Jelly point is 8 degrees above the boiling point of water, or 220 degrees here in NE Texas - but may be different for you based on your altitude.
NOTE: When processing time is less than 10 minutes it’s necessary to sanitize the jars beforehand. I put them in the water bath and sanitize them in the simmering water while I’m making the jam.
If this is your first time canning, it is recommended that you read Principles of Home Canning link at the bottom of this post. For more information on water-bath canning, please read the link at the bottom of this post Using Boiling Water Canners before beginning.