Wash and Sort: Dump your grapes into a clean sink filled with cool water. Discard any shriveled berries, leaves, or stems. You want only firm, plump fruit.
The Crush: Place the grapes in a large, heavy-bottomed stainless steel pot. Add about 1 cup of water (just enough to prevent burning). Using a potato masher, crush the grapes to break the skins.
Simmer: Bring the pot to a boil over medium-high heat. Reduce to a simmer, cover, and cook for about 10-15 minutes until grapes are soft.
Strain: Place a colander over a large bowl lined with cheesecloth. Pour the hot fruit mixture in. Let it drip naturally for at least 2 hours (or overnight). Do not squeeze the bag to keep jelly clear.
Measure: Measure exactly 5 cups of prepared juice into a large pot (6-8 quarts). If short, add a splash of water.
Dissolve Pectin: Whisk in the powdered pectin until dissolved. Add the half-teaspoon of butter now to prevent foaming.
Hard Boil: Bring the mixture to a full rolling boil on high heat (bubbles that don't stop when stirred).
Add Sugar: Quickly pour in all the sugar at once. Stir constantly. Return to a full rolling boil and boil for exactly 1 minute.
Skim and Fill: Remove from heat and skim off foam. Ladle hot jelly into sterilized half-pint jars, leaving ¼-inch headspace. Wipe rims and apply lids fingertip tight.
Process: Place jars in a boiling water canner covered by 1 inch of water. Process for 5 minutes. Remove and let cool for 24 hours.