Measure and make sure that your blocks are all the same size. Cut your sashing the same length of the blocks and use pins to match them up. You know that the sash is straight and true so if the block has bias edges or is a little stretchy from the seams, put it on the bottom so the extra can be eased in by the feed dogs. When sewing the long crosswise sashing to the row of blocks, I mark the block and sash measurement all along the crosswise sash and pin at the marks in the seam allowances.