Wash the chokecherries plain water and extract the juice by combining the fruit and water in a large saucepan. Bring to a boil over medium-high heat then reduce to a simmer over low heat. Gently mash the chokecherries with a potato masher to release their juices and continue simmering over low heat for 30 minutes, stirring occasionally.
Cover the canner and bring to boil over high heat. Once water boils vigorously, continue boiling for 10 minutes at altitudes of less than 1,000 ft. (adjust processing time for your altitude if necessary).