Find the center on all four sides of each piece. Pin it together there, and outward. The extra 5 inches on each corner of the backing will be at the corners. Then sew that triangle at each corner, which miters it.
Turn it right side out, and the self binding is that 5" that pulls around to make the edge (my friends was that yellow, which was actually the backing, which pulls around to the front for a 'border'.
When it's all turned right side out, smoothed out, then topstitch on the edge of the seam between front and back (by that 5" border) which holds it from shifting in use and laundering.
Does this help?