Separate the pierogi dough into two balls. Roll out one piece at a time on a lightly floured surface until about 1cm thick, but not too thin so that it tears. Cut into circles using a cookie cutter, pierogi cutter, or a glass 4-5cm in diameter. Brush a little water around the edges of the circles, and spoon some filling into the centre. Fold the circles over into half-circles, and press to seal the edges to create half moon shape. press edges with fork. Place pierogi on a cookie sheet, and freeze. Once frozen, transfer to freezer storage bags or containers.