Some good homemade plum jam recipes include one from AllRecipes.com and Natasha's Plum Jam, which doesn't require pectin. Another good recipe is Kraft's Sure.Jell Plum Jam that requires 6 cups of ripe plums, 1/2 cup of water, 1/2 teaspoon of butter, 8 cups of sugar of 1 box of fruit pectin. Combine the plums and water in a saucepan, bring the mixture to a boil, and simmer it for five minutes before stirring in the pectin, butter and sugar.
After boiling the jam for one minute, remove it from the heat, and skim off any foam. Spoon the jam into jars. To can the jam, first sterilize the jars and lids in boiling water, and then ladle in the jam. Process the jarred jam in a canner for 10 minutes.
To make Natasha's Plum Jam, halve and pit 12 pounds of ripe plums, and coat the plum halves with 4 1/2 cups of sugar. After allowing the mixture to sit for an hour, pour the plums and sugar into a pot, and bring the mixture to a boil. Reduce the temperature to a simmer for 10 minutes, and then allow it to cool to room temperature. Do this four times, stirring during the last round of boiling to keep the jam from burning to the bottom of the pot. Spoon the mixture into prepared jars.