Place the frozen berries and cranberries, sugar and 3 cups of water in a pot over high heat. Bring to a boil and reduce heat to medium-high and simmer for about 15 minutes.
Taste kissel and add more sugar if necessary. This will depend on the natural sugars in your fruit.
Add the potato starch to water in a small jug, and slowly pour into kissel. Stir constantly while kissel thickens (do not stop stirring!). If you want a thicker consistency, an extra tablespoon of starch to ¼ cup of water and repeat process.