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.
Wash nice large red Siberian crab-apples, cut out the stems, blossoms and all black specks and knots; add a small amount of water and steam or simmer slowly until the pulp is soft enough to press through a rather coarse sieve. Allow one-third as many plums as apples. Red plums are the nicest for this purpose, although damson plums may be used. Stew or steam the plums until tender. Pass through a sieve, and add to the crab-apple pulp.
Allow one pound of sugar to each pound or pint of pulp. Heat the sugar in the oven and add to the pulp after it has come to a boil. Cook slowly and steadily, stirring almost constantly until it is a thick jam.
Put into jars or glasses.