Wash the damson plums and remove any stems or leaves.
In a large pot, combine the plums, water, sugar, and lemon juice.
Cook the mixture over medium heat, stirring occasionally, until the sugar has dissolved.
Increase the heat to high and bring the mixture to a rolling boil, then reduce the heat to low and let it simmer for about 20 minutes, or until the plums are soft and mushy.
Remove the pot from heat and let it cool for a few minutes.
Pour the mixture through a strainer into a large bowl, pressing down on the solids to extract as much juice as possible.
If desired, add pectin to the strained juice according to the instructions on the package.
Pour the juice into sterilized jars and seal them tightly.
Store the jars in the refrigerator for up to 3 months or in the freezer for up to a year.