In a large bowl, toss the plums, sugar, and lemon juice.
Cover with plastic wrap and place in the fridge overnight (or at least 8 hours).
Place a small plate in the freezer right before cooking the plums (this is to use to check if plum jam is ready).
Place the plum mixture in a medium sauce pan on high heat. Bring to a boil for a couple of minutes. Lower heat to a low simmer. Stir with a wooden spoon and loosen any pieces that may stick to the bottom.
Be sure to keep stirring for about 15-20 minutes. You’ll know it’s ready when juices become thicken like a syrup. You could check temperature with a candy thermometer to make sure it’s never over 220 F. As it simmers, remove any white foam that could come to the surface.
To make sure the jam is ready, remove the small plate from the freezer. Place a teaspoon of jam on the plate. If it sets within a minute, it is ready. If it still appears runny, return the plate to the freezer and continue to cook the plum mixture. Test again in a few minutes.
Carefully spoon the jam into sterilized jars. Apply lids jars.
Wipe rims with a clean, damp towel and screw lids on securely but not too tightly.
Process in boiling water bath for 10 minutes.
Carefully remove jars using tongs. Use potholders to tighten the lids. Place jars on a rack upside-down and let them cool. When they’ve cooled down, store them in the refrigerator. The jam should keep unopened for several months.