After washing the chokecherries, add them to a large pot and cover them with an equal amount of water. Simmer the chokecherries for 30 minutes or longer. Once the fruit begins to burst, smash the berries using a potato masher. Keep in mind that if you simmer the fruit longer, more of the juice will evaporate, yielding less juice. Strain the liquid through a sieve or jelly strainer. If you prefer a clearer liquid, don't press down on the chokecherries in an attempt to get more juice out of the berries. This can cause your juice to be cloudy. Another great way to juice chokecherries is with a steam juicer.