I used to can them. I would fill a clean jar with clean plums and pour boiling water over them until they were within half an inch from the top of the jar. Seal and process. It has been so long I don't remember how long. The great thing is other than washing them well you don't have to do anything to them. I left the seeds in. As they sit they make their own very beautiful dark purple sweet syrup which we would pour over pancakes. I would let them sit for a couple of months prior to opening them. We do about the same thing to make concord grape juice. Ann in TN