Wash the Concord grapes.
Cut the apple into quarters -- peel, core, and all -- then chop coarsely. Set aside.
Put a couple of cups of grapes int a large stainless-steel pot, then crush them with a potato masher of the bottom of a clean glass jar. this provides a small amount of juice and prevents scorching.
Add the water.
Add the cut-up apple.
Heat the fruit mixture slowly to the boiling point, reduce the heat, and simmer until the seeds come free from the pulp.
Line a large colander with several layers of damp cotton cheesecloth. Set the colander over a large pot or bowl and carefully pour the grapes and liquid into it. Allow the free-run juice to drip through the cheesecloth. You may also use a chinois or jelly bag. Do not press down on the fruit.
Measure the free-run juice. Process into jelly 4 cups of juice at a time - a smaller batch means the jell point is reached more quickly, resulting in better flavor.
Taste a little bit of the juice. For every 1 cup of reasonably sweet grape juice, measure out 2/3 cup of sugar. If you used a greater percentage of under-ripe grapes and the juice is on the tart side, you can use 3 or 3 1/2 cups of sugar to 4 cups of juice.
Bring juice to a boil then add the sugar. Boil to the jell point.
Fill and process prepared jars.
