When I use sashing and cornerstones, I cut the sashing and the cornerstones for the project. I sew the sashing on the side of the blocks and sew the entire row together. Then I sew the row of sashing and cornerstones and attach it to the block row.
One thing to pay attention to is pressing the seams. Depending on which is light and dark, I press the cornerstone seams toward eachother and the corresponding sashing seams toward the blocks. That way they nest into eachother and become much easier to pin and sew. If the block is the lighter color, then I press the sashing toward eachother and the cornerstone the other direction.