Begin by sterilizing your jars and preparing a boiling water bath canner. Rinse the grapes thoroughly and remove the stems. Separate the skins from the pulp by squeezing each grape—set the skins in one bowl and the pulp in another. If desired, chop the skins using a food processor or knife for a finer texture.

Place the grape skins in a saucepan with about ½ cup of water and cook gently for 15 to 20 minutes, just until softened. In a separate pot, cook the grape pulp without any added water until it softens and begins to break apart. Once soft, press the pulp through a fine mesh sieve or food mill to remove the seeds.

Combine the strained pulp, cooked skins, and sugar in a large, heavy-bottomed jam pot. Stir well and bring the mixture to a boil. Continue boiling the jam for about 10 minutes, stirring frequently as it thickens to prevent sticking or scorching. Use the chilled plate test or a thermometer to check for gel stage (220°F at sea level).

Ladle the hot jam into hot, sterilized jars, leaving ¼ inch headspace. Wipe the rims with a clean, damp cloth and apply two-piece canning lids. Process in a boiling water bath canner for 10 minutes at elevations up to 6,000 feet or 15 minutes above 6,000 feet. Let jars cool undisturbed for 12 to 24 hours before checking seals.