Tutu Witch Costume - Willow, the Wild Witch - Black, Purple, and Zebra Sewn 10" Tutu & Witch Hat - sizes Newborn to 5T - Wings Not Included. $64.00, via Etsy.
One or two rolls of 6" wide tulle, no-roll elastic 1” shorter than kid’s waist, sewn together to make circle. Measure from waist to length you want tutu and double. (if 15”, cut 30” strips) Slip elastic above your knee & slip each strip of tulle under elastic to center of tulle strip & tie double knot. Continue until it’s as fluffy and full as you like.