Wash and coarsely chop plums, or leave small plums whole. Pit if freestone, or leave pits in if clingstone. Do not skin plums, the peels are needed as a pectin source.

Place plums in a stockpot or deep saucepan with 1/2 cup water per pound of plums (ie. 3 cups water for 6lbs plums).

Bring the plums/water to a hard boil over high heat, then reduce the heat and simmer for 15-20 minutes until the plums have completely fallen apart.

Remove from heat and strain the juice through a jelly bag or fine mesh strainer lined with a double layer of cheesecloth. Allow the mixture to strain for at least 1-2 hours, or overnight until the pulp is quite dry and the juice is extracted.

Measure the extracted juice. For every cup of plum juice, add 3/4 cup sugar. Starting with 5-6 lbs whole plums you should have about 4 cups juice (so add 3 cups sugar), but the yield will vary depending on the juiciness of your fruit.

Bring the sugar and juice to a hard boil in a deep saucepan. Cook over high heat for about 15 minutes, until it reaches gel stage. Test for gel stage with an instant-read thermometer (220 degrees F at sea level, see notes for altitude adjustments). Or, test for set on a plate that's been chilled in the freezer.

When ready, ladle the plum jelly into prepared jars leaving 1/4 inch headspace. Wipe rims and seal with 2 part lids.

If canning, process the jars in a water bath canner for 10 minutes (below 6,000 feet in elevation) or 15 minutes above 6,000 feet.

Properly water bath canned jelly will maintain peak quality on the pantry shelf for 12-18 months. Store in the refrigerator after opening. If not canned, allow the jelly to cool to room temperature and then store in the refrigerator for up to 3-4 weeks, or freezer for 6 months.