"Why will I ever in my life need to know the formula for the circumference of a circle!?!" Little-Squirrel asks.
"Well one day," answers Big-Squirrel from the future, "in the distant future, you will want to make yourself a skirt and that darn formula will help you calculate the waist and the diameter of a whole skirt!"
Little-Squirrel just rolled her eyes. "Sure, whatever."
I measured my waist (plus 2 inches) for the half-inner circle; only half the circle is my whole waist measurement. And then doubled it to get a whole circle's circumference.
The waist band was two long rectangles. I attached them to the slightly gathered skirt-waist by sandwiching the skirt between the waist pieces (right sides facing in), stitching, folding them up and hand stitching the top edge (I didn't want any top stitching to show).