Align the darkest rectangle right side down, with its long edge across the (top) seamed end of the first two patches. Lengths should match exactly.
Sew a seam along the entire length of the patches as shown above. Press the seam allowance towards the rectangle.
Add the third (medium dark) rectangle in the same way, sewing it to the (side) edge created by the second rectangle and square. Press the seam allowance towards the rectangle.
Moving in the same direction, sew on the fourth rectangle, matching it lengthwise with the (bottom) edge created by the square and third rectangle. Press the seam allowance towards the rectangle.
Now finish sewing the partial seam. Fold the first rectangle towards the right to expose the seam between it and the square. Align its edge with the remaining portion of the square and the rectangle below.
Begin stitching just before the spot where the partial seam ended. Backstitch, then stitch to the end of the patches.
Press the seam allowance towards the center square. That's it!