Cook the grapes in a large saucepan for about 15-20 minutes so that a lot of juice comes out. Press grapes with juice through a sieve so that the pulp separates from the seeds. Bring the grape puree to the boil with the cinnamon stick and stir the sugar into the boiling puree. Let cook for about 5 minutes. Then pour into preserving jars, close tightly and let them stand upside down for about 5 minutes.