Reduce the heat and simmer, covered, for 5 minutes. Simmer for another 5 minutes while stirring occasionally.
Plums are ready when the skins burst.
Press the plums through the Stainless Steel Food Mill into a small bowl. Measure the plum pulp. For wild plums add 1 cup sugar for every 3 cups plum pulp. For domestic plums add cup sugar for every 3 cups plum pulp. Stir to blend.