Begin by preparing a small boiling water bath canner. Add enough water to the bottom of the canner to cover the jars (about 3 or 4 inches to start - you can add more after you’ve loaded the jars). Turn the burner on to medium heat.
While the water is heating up, you can sterilize your canning jars and bands. Remember to always start with fresh lids and not to reuse old ones, though you can always reuse bands and jars. Sterilize the lids in a saucepan of water over the stove, allowing them to simmer while you sterilize your jars and bands.
Chop or crush the plums, removing the pits until you have just the pulp left.
Add the plums to a saucepot, adding your sugar as you do so. Stir.
Allow the sugar to dissolve, then set the pan on the stove over high heat and bring to a boil.
Cook the mixture, stirring often for about 15 to 25 minutes. The fruit should thicken and reduce by about a third. Add more or less time as needed.
Taste the mixture and add lemon juice if it is too sweet for your liking.
Once the jam is thick and has a glossy appearance, you can remove it from the heat.
Funnel the jam into the jars, leaving about an inch of headspace. Wipe the rims to remove any debris or food particles that may have spilled on there. Your jars should be hot when you load them. This will prevent them from cracking when they are placed in the hot water inside the water bath canner.
Put the lids and bands on the jars, screwing them on until they are fingertip-tight.
Load the jars into the canner. If the water does not cover the jars, add more until it does.
Bring the water to a rolling boil. Once the water has started to boil, start timing. You will process the jars for 10 minutes.
After ten minutes, use a pair of tongs or a set of jar lifters to remove the jars from the corner. Place them on a kitchen towel to cool. As they are cooling, keep them out of any drafts. This can cause the jars to crack.
As the jars cool, you will hear them making popping noises. This is a sign that the jars are sealing. Regardless, after 12 to 24 hours of cooling, check each lid to make sure it has sealed onto the jar.