This section is from the book "Preserving And Pickling", by Mary M. Wright. Also available from Amazon: Preserving and Pickling: Two Hundred Recipes for Preserves, Jellies, Jams, Marmalades, Pickles, Relishes and Other Good Things.
The yellow or green-gage plums, or the red plums are good for preserves, although any kind may be used. Scald the plums with boiling water, and remove the skin. Halve, and seed. Weigh the fruit and allow a pound of sugar to each pound of fruit. Place the fruit in a preserving kettle, then let it stand overnight. In the morning pour off the juice and boil to a syrup, then add the fruit, and simmer very slowly until clear. Lift the plums carefully out with a strainer and fill into glasses or jars; then when the syrup has been boiled to the consistency of jelly pour over the fruit, and seal up while hot.