Local plums are not in season too often so it is always nice to preserve something and enjoy plum flavor and nutrition even when not available.
All you need is to remove the seeds, cook with some water, sugar/sweetener, lemon zest, cinnamon sticks and maybe fresh thyme sprigs (optional).
Cook it down for about 20 minutes, taste. When fruits get cooked they usually become very tart/sour, you want to balance that with the sweet without making it too sweet. Keep tasting and adjusting where necessary.