Wash the plums thoroughly.
Start by removing the pit from the plums.
Cut the plums into halves or quarters. Don't worry about uniformity, as they'll break down during cooking.
Place the cut plums in a large pot.
Heat the pot over medium heat.
Stir occasionally as the plums begin to release their juices and rise to the surface.
Once the plums are bubbling, reduce the heat to low.
Continue cooking, stirring frequently to prevent sticking.
Cook until the volume has reduced by half and the mixture has a thick, paste-like consistency.
Remove from heat and let the jam cool while you prepare the jars.
Wash the jars and lids thoroughly with warm, soapy water.
Rinse the jars and lids well to remove all soap residue.
Place the clean jars and lids on a metal tray.
Put the tray in the oven and set the temperature to 150°C (300°F).
Leave the jars in the oven for 12-15 minutes, or until completely dry.
Carefully remove the hot jars from the oven using oven mitts.
Place each jar on a heat-resistant surface (the back of a knife works in a pinch).
Fill each jar with the warm jam, leaving a small space at the top.
Securely put the lids on the jars.
Wrap the filled jar tea towels and allow the jars to cool slowly at room temperature overnight.
Once cooled, store the jars in your pantry.