See sauerkraut recipe.
Fill with one three inch dough circle with a tablespoon of the sauerkraut into the center of the circle and fold it in half. Using your fingers, tightly pinch the edges together to seal and create a 1/2-inch border. Arrange the filled pierogi on a lightly floured surface or large rimmed baking sheet and dust very lightly with flour; loosely cover with plastic wrap or a clean dishtowel. Repeat with the remaining dough circles and filling.