If I can, I sew the vertical sashing between the blocks. I then use one solid piece of sashing on the horizantal sashing strip. If that method won't work, I do the cornerstones in betweeen the blocks/sashing but I also add the triangle pieces (square and sew and flip out) to the sashing so that where the sashing meets the cornerstone it forms a star. You don't notice if they are off a little because of the star effect.