Cut the watermelon peel and the plums s into small pieces and pour them into a large saucepan.
Add sugar, cinnamon and mix well.
Put on fire and, over a high heat, reach the boil.
Lower the heat and cook the jam for about 50 minutes, stirring occasionally.
Meanwhile sterilize the jars and the caps: you can boil both in water and leave them to dry on a clean cloth, or you can boil only the caps and pass the washed and still wet jars for 2 minutes in the microwave oven at maximum power.Take back the jam and make a test to verify that the jam has the desired consistency: with a spoon take the most liquid part of the jam and pour it on a coffee plate.Put it in the freezer for 2 minutes, then tilt it and verify how long the drops takes to go down: if they run down immediately, like water, it means that it’s necessary others 5-10 minutes of cooking, if they run slowly the jam is perfect and ready!
Pass the jam a little with the immersion blender to make it more homogeneous.
Immediately pour the hot jam into the jars, close with the caps and turn them upside down for half an hour.
Then proceed with the double sterilization putting the jars in a large pot and covering them completely with water: bring on fire, reach the boil and cook for 20 minutes. Turn off the heat and let the jars cool in the water.
Drain them, label and store!