Use a scrap piece of paracord to wrap around the widest section of your head. Measure the distance between where the paracord meets the end. Divide that by two. For example, I ended up with 11". Add a half inch so it's not too hard to take off. If you want to add a safety release (look at step 8) add a full 2 inches. I got 1'1"
Take your 12' long piece and fold it in half. Measure the length you got from the scrap cord from the fold point and pinch with your hemostats at that point. You cannot use needle nose pliers for this. If you don't have hemostats, you can tie a knot around that point with some thread tightly.