So here is how I did this little number. Wish I had a pic of me wearing it because it looks funny on the mannequin with no shirt/discernible waist...SO super quick here's how:
Waist measurement = 35"
I wanted to double the gathering so it was nice and full
Cut 2 bid squares 35in x 18" (length of skirt)
Cut 2 small rectangles 35in x 9in (waistband height x2)
I made a mini (18" skirt length) but you can do down the knees (about 20-22")
sew waistband pieces together at short ends to form big circle. Fold in half so it is 4.5" tall, press. Mark center and sew an elastic casing all the way around circle 1/4" wider than elastic. Press seams.
Sew skirt pieces together at side seams. Press seams.
Sew skirt to waistband. Press seam down.
Use seam ripper to open one waistband side seam inside elastic casing. Insert elastic (same measurement as waist) and sew ends together.
Hem at desired length. A wider hem will help weigh down the full skirt (I did a 2" hem).