I square up as I go. Every time I sew a section of the block together. Especially the 1/2 square triangles. ( And always make them slightly larger to allow for squaring.) And after the block is finished, I press and square up the block. All the blocks have to be the same size and allowing for the 1/4 inch seam allowance. A 12 inch finished block will be 12 1/2 inch before sewing the rows together.