I use the method of adding left, then bottom sahing whenever am using corner stones. Cut all sashing pieces the size of the block. Sew the left side sashing on the block. Sew the cornerstone to another piece of sashing. Sew the cornerstone and sashing piece to the bottom of the block, making sure the cornerstone is sewn to the side sashing. It is much easier than fighting lining up the sashing 'cornerstone sashing cornerstone, etc' long piece when putting rows together.