As many plums as you can handle
As much syrup as you need
As many jars as it takes
Optional: any flavorings you fancy to add excitement to the fruit (vanilla beans, rosemary sprigs, etc.)
Sterilize your jars, either in boiling water or a dishwasher. Wash the plums, and prick them a few times with a skewer. This allows the syrup to permeate, and prevents the skins from bursting (although if you have a particularly thin-skinned variety of plums, they might burst anyways, leading to a still-delicious-albeit-somewhat-less-pretty product). Place the plums in your canning jars, tamping them down to fit as many as possible. They'll shrink in the hot syrup, so really pack them in. Tuck any desired flavorings in amongst the fruit.
Prepare your syrup: I favor a medium syrup, of 2 parts water to 1 part sugar. Make as much as you'll need to fill your jars. Bring to a boil, and then pour over the plums, up to 1/2" of the rim.
Top jars with sterilized lids, screw the rings on finger-tight, and then process in a boiling water bath (20 minutes for pints, 25 minutes for quarts). Remove and cool, then check that the lids have sealed. The syrup will infuse the plums (and vice versa) as they sit. By winter, they'll be amazing.