Sterilize your mason jars (pints or quarts) with boiling water. You can put them in a large pot of boiling water, or you can set them in a clean sink and pour boiling water into them. Once they're sterilized (give them 5 minutes in the hot water), pour out the water or remove them from the pot of water, and put them face down on a clean towel on a counter. Put the lids in a saucepan full of water, bring to a boil, and turn off the heat.

Fill your canner halfway or a bit more with water and bring to a boil.

Mix your sugar and water together and heat to simmering. Use 2 cups sugar to 5 cups water. If you're making 6 pints, make about 4 pints of this. 

Alternatively, you can heat apple juice instead of syrup.

Wash the plums, slice vertically, and remove the pits. Now stuff them into the jars, leaving about 1 inch space at the top. Add your star anise, orange zest, cinnamon, and whiskey now, if using.

Pour in the hot syrup or apple juice, making sure to cover the plums.

Poke a spatula down the inside edge of each jar to release bubbles, clean the rims of the jars, and put the lids and rings on. Screw down the rings, but not overly tightly. You should screw the rings down firmly, but not so that they're hard. to open.

Process the jars in the boiling water canner, 20 minutes for pints, 25 minutes for quarts. If you live at a higher altitude than 3000 feet, you'll have to add time.

Using canning tongs, remove your jars from the canner and set them on the counter, with a cloth or cutting board beneath them. 

Leave them out for 24 hours, then check to see if they've sealed. The lid should be concave, not bulging upward. If they've sealed, remove the rings, label, and store. If the top comes off easily once the ring is removed, the jar didn't seal, in which case you can freeze it (make sure the jars are not full when you freeze them, or they'll burst in the freezer), or refrigerate and eat it soon.