I pit the plums and grind them in a food processor until they are small pieces. I then cook them down with sugar. Sorry there isn't any recipe because the amount of sugar depends on how sweet the fruit is. Cooking the fruit down is a long process I've never have tried to speed up the process by adding pectin. I am a purest. The jam is done when it coates a spoon and holds together.
Make sure you follow the boiling water bath for sealing the jars.