Pit 1 quart of sour cherries and chop them (to make about 2 to 2–1/2 cups fruit). Add sour cherry juice (see note) to make a total of 6 cups fruit and juice. Place fruit, juice and a 57g package of pectin in a large pot. Bring to a rolling boil, add 5 cups sugar and let boil for 10+ minutes until it’s thick enough to set (see note). This yields 8 pints. Place in canning jars and process for 10 minutes in water bath.
Note: I freeze sour cherries. When defrosted they give off too much juice for pies. I re-freeze the excess juice and save it for making jam. Regarding how long to boil the mixture, there are a variety of tests you can perform, but for reference, my jam reached 224 F (I’m at sea level).