Stem a mixture of ripe and green mulberries until you have a quart [4 cups]. Cover them with cold, salty water (1/4 c. salt per quart of water). Let set 5 minutes. Then drain, and rinse in cold water several times. Now crush the berries. Add 3 c. sugar, 1/4 c. lemon juice (or 1/4 c. cider vinegar like the old-timers), and 1/2 t. cinnamon. Cook over low heat, stirring until the sugar dissolves. Then boil hard, stirring constantly, until it arrives at the jellying point. Take off heat. For the next 5 minutes, alternately skim and stir. Have ready clean, hot jars. Ladle your jam into them, and put on lids. Process in a boiling water bath for 5 minutes.
I used sugar in my jam. I also froze it instead of doing a boiling water bath.
I got this recipe from The Encyclopedia of County Living by Carla Emery.
