Have you tried the technique used in Paper Piecing? By drawing precise lines, you will know exactly where to place your Dark Diagonal pieces of cloth against the light vertical rows. You could first secure all your vertical stripes with a basting stitch so they won't move. Then, carefully draw your lines using a yardstick, then place your dark pieces in place. Snip the basting stitches where needed. Then, re-stitch all the vertical seams. Good luck, do give us a photo of your finished work! I'm subscribing to your thread, because I am interested in your results.