It's a lot easier to do sashing without cornerstone if it's your very first try or if your blocks are a bit out of square. FIRST SQUARE UP THE BLOCKS! Cut sahing width as wanted, plus 1/2, and sew to each side of each block (except the ends). Cut these pieces all the same to square your blocks. Then sew blocks together as a row. Do all rows and measure carefully because you're going to cut the long strips of sashing all the same length (average length of the rows is how I do it) to keep everything square. You'll have to ease in the extra fabric to make them exactly even on the ends. The importance of squaring applies even if you use cornerstones