To extract grape juice, stem the grapes into a stockpot or saucepan and add 1/2 cup water.

Bring the mixture to a boil while mashing the grapes to encourage them to release their juice. Continue to cook until the grapes completely fall apart, about 10 minutes.

Remove from heat and strain through a jelly bag or double layer or cheesecloth. Allow the juice to strain for at least 2 hours.

Refrigerate the juice overnight to allow tartrate crystals to separate out.

Strain the cold juice through the cheesecloth again. Pour carefully, and leave the tartrate crystal sediment behind at the bottom of the jar. Not all grapes form visible tartrate crystals, but this is a good precaution anyway and will prevent crystals in the finished grape jelly.

If you're starting with purchased grape juice, skip these steps and begin below.

Add 4 cups of grape juice to a large heavy-bottomed saucepan, ensuring that the fills the pot no more than 1/3 of the way (mixture will foam). Add the powdered pectin and stir to dissolve. (Don't add the sugar yet.)

Bring the mixture to a boil over high heat and boil 1 minute.

Add the sugar and stir to dissolve. Bring the mixture back to a hard boil and boil 1 minute before turning off the heat.

Ladle the grape jelly into prepared jars, leaving 1/4 inch headspace. Store in the refrigerator for a refrigerator jam and use within 3-4 weeks.

If canning, process in a water bath canner for 10 minutes if under 1,000 feet in elevation. Add 1 minute to the processing time for every additional 1,000 feet.

Check jar seals after 24 hours, and store any unsealed jars in the refrigerator for immediate use. Properly canned and sealed jars should maintain quality on the pantry shelf for 12-18 months.