I tend to sew by rows rather than columns.... So that means I sew the vertical sashings on first and sew all that row together. The next row would be the horizontal sashings and any cornerstones if used.
I then put two rows together at a time and build from there. I don't just sew 1 to 2 and then that to 3...I sew 1 to 2, 3 to 4 and 5 to 6...etc