Place a small plate in the freezer right before cooking the plums (this is to use to check if plum jam is ready).
Wash the plums and remove any blemishes. Cut the plums in half, remove the pits, and cut the pieces in half again.
Place a large skillet on medium-high heat. Add the plums, sugar and a 1/4 teaspoon of salt. Using a wooden spoon, stir together to combine. Bring the mixture to a boil and keep stirring until the sugar dissolves. Mash the plums to break them up with the spoon (or a potato masher).
Add in the lemon juice and keep stirring. Use your spoon to loosen any pieces that stick to the bottom of the pan.
Lower the heat to a simmer and keep stirring until the bubbles will slow down. As it simmers, skim off any white foam that comes to the surface.
Be sure to keep stirring for about 15-20 minutes. You’ll know it’s ready when juices thicken like a syrup and when you lift some jam up with your wooden spoon while stirring, it will fall of the spoon in sheets. That's the main sign that the jam is ready.
Another easy way to check if 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.
When the jam is ready, let it for 5 minutes to allow the fruit to settle
Carefully spoon the jam into sterilized jars. Leave about 3/4 inch of headroom.
Let the jars cool completely. Seal tightly with the lids. Label and date the jars. Store in the refrigerator for up to a month. You could freeze for up to year.