Remove the peel from the plums and remove the seeds; in the ingredients, the weight of the plums is already seedless. The skin can be left but without it the jam is more tender.
Mash the plum with mashed potatoes and sprinkle with sugar.
Put on the fire, dissolve sugar and cook after boiling for 1.5 hours, stir occasionally so that the jam does not burn.
Put the finished jam in sterile jars and roll it up.