Prepare canner, jars, and lids. Wash your jars and lids. Sterilize your jars in a large pot filled with water. Bring the water to almost a simmer over medium heat. Keep the jars in the simmering water until you are ready to fill them. Use a jar lifter to remove them from the water when you are ready to fill them, dumping the hot water back into the pot when you remove each jar from the water. Right before you are ready to can, put your lids in a small pot or bowl with hot, but not boiling water. Put some additional white vinegar in a small bowl, alongside a clean washcloth or paper towel. Keep the water in your canning pot at a simmer while you fill your jar. See The The All New Ball Book of Canning and Preserving for additional guidance

Once simmering, continue to cook for 7 minutes until the plums release their juices.

Pour the plums into a colander placed over a bowl, gently stirring the plums to further release any juices.

Return the juice and the rosemary sprig back to the pan and bring to a boil over high heat. Gently boil for 10 minutes, stirring occasionally, until the syrup is thick and reduced.

Remove the rosemary sprigs (or other added spice)

Simmer for about 15 minutes, stirring frequently until a small dab of the jam placed on the plate in the freezer yields slightly when pushed.

Prepare your work surface On a clean cloth, lay out your jar lifter, bubble remover/lid lifter, funnel, bowl for lids in hot water, bowl of vinegar and clean cloth.

Remove the jam from the heat and place on trivet where you will be canning. (If you will not be canning the jam, fill your jars, allow them to cool and then refrigerate.)

Spoon your jam into hot jars, leaving 1/4 inch headspace. Headspace is measured by laying your headspace gauge on the rim of the jar. In this case, have the notch next to '1/4 inch' laying on the rim of your jar. Your jam should just touch the bottom of the gauge.

Screw band down until resistance is met, then increase to fingertip tight. Fingertip tight is defined as tightening just with your fingertips, not needing the palm of your hand to tighten.

Place jars in canner, making sure they are completely covered with water by 2 inches.

Cover pot and bring to a boil.

Once boiling set your timer and boil for 10 minutes.

Carefully remove jars with the jar lifter and place them on a rack where they can sit undisturbed for 24 hours.