Step 1: Prepare the Plums

1. Rinse the plums under cold water and remove any stems or leaves.
2. Cut the plums into quarters, leaving the skins and pits intact. The skins and pits contain natural pectin, which helps the jelly set.

Step 2: Cook the Plums

1. Place the plum pieces in a large pot and add the water.
2. Bring the mixture to a boil over medium-high heat, stirring occasionally.
3. Reduce the heat and let it simmer for 20-25 minutes, or until the plums are soft and the mixture is juicy.

Step 3: Strain the Mixture

1. Place a fine-mesh strainer or cheesecloth over a large bowl.
2. Pour the cooked plum mixture into the strainer, pressing gently with a spoon to extract as much juice as possible. Discard the skins and pits.

Step 4: Prepare the Jelly Base

1. Measure out 4 cups of plum juice and pour it into a clean, large pot.
2. Stir in the lemon juice and powdered pectin (if using) until fully dissolved.

Step 5: Boil the Mixture

1. Bring the plum juice mixture to a rolling boil over high heat, stirring constantly.
2. Add the sugar all at once, stirring until it dissolves completely.
3. Return the mixture to a rolling boil and let it boil for exactly 1 minute, stirring continuously.

Step 6: Test for Doneness

1. To check if the jelly is ready, perform a spoon test: Dip a cold spoon into the mixture and let it cool slightly. If the jelly coats the spoon and forms a gel-like consistency, it’s done.

Step 7: Jar the Jelly

1. Remove the pot from the heat and skim off any foam from the surface.
2. Carefully ladle the hot jelly into sterilized jars, leaving 1/4 inch of headspace at the top.
3. Wipe the rims of the jars with a clean, damp cloth to remove any spills.
4. Seal the jars with sterilized lids and rings.

Step 8: Process the Jars (Optional)

1. For long-term storage, process the jars in a boiling water bath for 10 minutes.
2. Remove the jars and let them cool completely on a towel. Check the seals by pressing the center of the lids—if they don’t pop, the jars are properly sealed.