Light Blue 100% Nylon Tulle.
How to gather a tutu: Get a solid, slightly thicker thread (ideally crochet thread but topstitching or dental floss work), it needs to be strong. Just leave it on the spool, let it unwind. Set your machine to a medium width and LONGEST zigzag, then lay the thread down and zigzag OVER it (not on it) – the thread needs to slide. Zigzag down the entire thread (and your fabric), take off the machine, then use the thread to pull up the gathers. It should slide very evenly over the thread. When you make the seam (attach to the waistband), get that gathering zigzag (and the thread) inside the seam. After the waistband is on you can pull the heavy thread and trim the zigzaging off if required.