Step 1: Wash grapes thoroughly and remove them from their stems. Discard any bruised or shriveled grapes.
Step 2: Place the clean grapes and 1/2 cup of water into a large, heavy-bottomed pot.
Step 3: Bring the mixture to a boil over medium-high heat. Reduce heat to low, cover, and simmer for 15-20 minutes, stirring occasionally, until grapes are very soft and have released their juice.
Step 4: Set up a strainer by lining a colander with several layers of cheesecloth or a jelly bag. Place the colander over a large bowl or pot.
Step 5: Carefully pour the cooked grape mixture into the cheesecloth-lined colander. Allow the juice to drip through naturally for several hours or overnight. Do not squeeze the bag to avoid cloudy jelly.
Step 6: Measure out 4 cups of the clear grape juice. If you have less, add a little water to reach 4 cups. If using powdered pectin, sprinkle it over the cold juice and stir until dissolved. If using liquid pectin, add it directly after measuring the juice.
Step 7: Pour the measured grape juice into a clean, large, heavy-bottomed pot. Add the granulated sugar. If using liquid pectin, ensure it’s already mixed in.
Step 8: Bring the mixture to a full rolling boil over high heat, stirring constantly to dissolve the sugar. A rolling boil is one that cannot be stirred down.
Step 9: Boil vigorously for 1 minute (for liquid pectin) or according to powdered pectin instructions. For no-pectin jelly, continue boiling until the mixture reaches 220°F (104°C) at sea level, or passes the cold plate test (a spoonful on a chilled plate wrinkles when pushed).
Step 10: While the jelly is cooking, sterilize half-pint canning jars and their lids. Wash them, then boil jars for 10 minutes or use a sanitizing dishwasher cycle. Keep them hot until ready to fill.
Step 11: Carefully ladle the hot grape jelly into the hot, sterilized jars, leaving 1/4 inch headspace. Wipe jar rims clean with a damp cloth.
Step 12: Place hot lids on the jars and screw on the bands finger-tight.
Step 13: Process the filled jars in a boiling water canner for 10 minutes (adjusting for altitude if necessary).
Step 14: Carefully remove jars from the canner and place them on a towel-lined surface. Let them cool undisturbed for 12-24 hours. Check for proper seals; if the center of the lid doesn’t flex, it’s sealed. Store in a cool, dark place.