maviskw

That looks good, but I would add only one of the cornerstone colors to the sashing piece.

You sew a sashing strip to the block on one side, then add a sashing strip with corner stone attached to a second side, with the cornerstone touching both sashing strips. All blocks are sashed and cornerstoned like this, and then they are all sewn together in rows. Now two sides of your quilt will have no sashing or cornerstones. You could add them on each block that needs them as you do your rows or add them in one long row at the end.