Originally Posted by Prism99

Create your horizontal rows. Attach a long sashing strip to the bottom of the first row. Iron, then turn it over to the wrong side. Use a pencil and ruler to mark the seam lines into the sashing strip. These become the sashing's "virtual" seam lines. When you attach the next row to this sashing strip, first match up and pin each seam to the marked line in the sashing strip. As you sew, ease or stretch the blocks as necessary to make sure the seams are lined up with the markings. This ensures that the rows line up when you are done.