Since this wall has diamonds nicely lined up in the corner, I'll bet that's where they started. It's really a matter of snapping (or taping) diagonal lines on the wall. Nicely done!

Start in the most visible corner ( the one that will be seen or noticed most). Decide on the size diamond you want and then make it an easy measurement for your room dimension. First paint the entire wall with your lighter diamond color.Then snap chalk lines and tape it off and paint your darker diamonds. Hope that helps. These diamond were approx 14"x20".