Cup of Yum
1. Rinse the plums, halve them, and remove the pits.
2. Add the plums to a large mixing bowl and top with the sugar, stir to coat and let them sit for 2 hours.
3. Transfer the mixture into a large cooking pot.
4. Over medium heat, bring it to a boil uncovered, and stir occasionally.
5. Add lemon juice and lemon zest.
6. Boil until the mixture is bubbling uniformly.
7. 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.
8. Last time bringing it to a boil, make sure to stir frequently to prevent scorching and do NOT cool it.

Sterilize The Jars:

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

Canning:

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