I always use corner stones. If I have a 12.5 inch block I cut a bunch of 12.5 x 2.5 sashing strips for the bottom of my blocks. Then I cut a 12.5 inch with of fabric ( sashing fabric) and then sew a 2.5 inch wof of my corner stone fabric to the bottom of my sashing fabric, press, then sub cut this into 2.5 inch sashing with the corner stone. I then sew one piece to the right side of my blocks. Then squre up if it's necessary. HTH.