Wash and pit the plums. I find a cherry pitter works well for this, since the plums are about the same size as cherries. A lot of pulp gets left on the pits, so I push them through a mesh strainer to get as much of that as I can.
Cut the fruit in halves or quarters. You can measure the volume of plums you have at this point. It should be about 5 cups or so.
In a large heavy-bottomed pot, mix the plums with the sugar. Let stand for 15 minutes. They will become very juicy.
Put a small plate or saucer in the freezer to use for the jell test later.
If you are canning your jam (instead of freezing it), sterilize your clean jars, lids, and bands in boiling water for 10 minutes. You want your jars, etc. to still be hot when you ladle the jam into them, so if they are ready before the jam, just leave them in the hot water.