• Rinse the plums, halve them, and remove the pits.
• Add the plums to a large mixing bowl and top with the sugar, stir to coat and let them sit for 2 hours.
• Transfer the mixture into a large cooking pot.
• Over medium heat, bring it to a boil uncovered, and stir occasionally.
• Add lemon juice and lemon zest.
• Boil until the mixture is bubbling uniformly.
• Simmer for 2 hours on low heat, then turn off the heat. Cool to room temperature. Repeat this step 4 times. I usually get this done over a period of 2 days, or I start early in the morning.
• Last time bringing it to a boil, make sure to stir frequently to prevent scorching and do NOT cool it.

Sterilize The Jars:

• Wash the jars and the lids with warm water and soap then let them dry completely, this is important.
• Boil the lids for 5 min.

Canning:

• Transfer the hot jam into the jars using leaving about 1/2″ space.
• Screw the lids on to hold a tight seal in place but don't over-tighten them.
• Place the jars into the canning pot and cover with 2 inches of water. Bring to a boil and process for 15 minutes.
• Remove from the pot and leave at room temperature undisturbed for 24 hours.
• After 24 hours, check that the seal has formed by pushing down on the center of the lid, it should not move at all.
• If the seal does not form, refrigerate jam and enjoy within 3 months.