Sterilize 5 half-pint jars and their lids. Prepare a hot water bath canner for making jam on the stovetop. Place the jars in a warm oven. Line a portion of the countertop with a kitchen towel.

In a small saucepan, bring the port to a boil over medium-high heat (this will take about 5 minutes). Reduce the heat to medium and let simmer for about 15 minutes, until it has reduced to ½ cup. Remove from heat and set aside.

Combine the cherries, lemon juice and pectin in a 4-quart saucepan. Use a potato masher to crush the cherries, then place the saucepan over medium-high heat and, stirring continuously, bring the mixture up to a full rolling boil. The mixture should continue to boil when stirred.

Add the sugar and reduced port and stir to combine. Bring to a full rolling boil once more. Let the jam cook at a hard boil for 1 minute, then remove the pot from the heat. Let the jam settle for a minute and, if necessary, use a spoon or ladle to skim off any foam.

Using a jar funnel, while the jam is still hot, ladle the jam into the jars, leaving ¼ inch of headspace at the top of each jar. You’ll have about ½ cup of jam left over—ladle it into a container to keep in the fridge. Wipe the rims of the jars clean, then screw on the lids and bands just until they are secure—be careful not to over-tighten the bands. 

Bring the hot water bath to a boil. Place the jars in the boiling water  and process for 10 minutes. Use a pair of jar tongs to transfer the jars to the towel-lined countertop. Let cool for 24 hours, then remove the bands and rinse any excess jam off of the jars with hot water. 

Let the jars fully dry, then store in the pantry for up to 1 year. After opening, use the jam within 1 month.