If you haven't done sashings before, here is a simple way to get everything to line up.
Make rows, with sashing in-between blocks. Add a long sashing to the bottom of the first row. Before adding the second row of blocks, turn that first row over to the wrong side and with a pencil and ruler extend the seam lines of the block across the long sashing. These pencil marks become your "virtual" seams.
When adding the second row, pin the seams in the second row with the "virtual" seams you marked in the sashing. Sew, easing or stretching as necessary to keep the pinned seams matching.
If you mark like this, you do not need cornerstones to keep the blocks lined up.