Rinse the plums, remove the seeds, and slice them into quarters.
Place the plums into a pot and cover them with sugar. Allow them to sit in the refrigerator overnight to develop a nice juice.
The next day heat the plum mixture over low heat on the stovetop, stirring it constantly to prevent the fruit from burning. Once it begins to boil, immediately remove it from heat. Repeat this process 5-7 more times, letting the plums completely cool between each round (this process will help to thicken the jam).
Once you bring the plums to their last boil, transfer the jam into jars or can them for longer use.