In a large saucepan, add in all the ingredients. Don't worry about the skins of the plum. They will break down some while cooking.
Turn the heat to medium and stir the ingredients together. As the plums warm up, they will form juice. You can smash them with the back of your spoon against the saucepan to help break them down.
Stir the ingredients occasionally. As the jam cooks down, it will thicken. When you run your spoon across the bottom of the pan and it briefly separates, you're ready to remove the jam from the heat. My jam took about 35 minutes to thicken to this stage. Remove the cinnamon stick.
Pour the jam into a mason jar. It should make about 1 and ¼ cups of jam. It will continue to thicken as it cools.