Wash, core and cut the plums into small pieces. It should make about 1.2 kg of pulp.
Scald walnut kernels with boiling water and then peel off the skin with a sharp knife. (This is a job that drives you crazy, I did it, but I think you can also use it with skin to avoid late damage to your own nerves!) Then break it roughly. Mix together with the plums, sugar, citric acid and plum water in a saucepan. Bring everything to the boil while stirring and let simmer for 3 minutes. (Gel test!)
Now fill the jam full to the brim in jars, close and turn upside down, allow to cool. Good succeed!