Prepare a boiling water bath canner and enough jars to hold 4 pints of product.
Cut plums in half and cut each half into 4-6 wedges.
Pile sliced plums into a low, wide pan, add the sugar and apple cider vinegar, and stir to combine.
Bundle the spices up in a length of cheesecloth or tuck them into a non-reactive tea ball and add them to the pan.
Place the pan on the stove and bring it to a boil. Cook at a vigorous bubble for 5-6 minutes, until the syrup thickens and the fruit softens (but try to stop cooking before the slices start to disintegrate).
Using a slotted spoon, funnel the fruit into jars and then top each jar off with the syrup, leaving about 1/2 inch of headspace.
Tap jars to loosen air bubbles and adjust syrup levels, as needed.
Wipe rims, apply lids and rings, and process jars in a boiling water bath canner for 20 minutes.
When time is up, remove jars from canner and place on a folded kitchen towel to cool.
If jars begin siphoning their syrup when you first remove them from the canning pot, pop them back into the water. Slide the pot off the hot burner and let the jars cool more gradually in the pot.
Once jars have had a chance to cool to room temperature on their towel, check the seals by pushing on the lids. If they are firm and without wiggle, the seals are good.
Sealed jars are shelf stable for at least a year.