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.
Bring a large pot of salted water to a boil. Drop in the pierogi, taking care not to crowd them too much (you might have to cook them in batches). Reduce the heat slightly so the water simmers and cook them until they rise to the surface; about 3-5 minutes.