You're going to need 2-8 yards of tulle, found in your local fabric store. Tulle seems to range in price from very cheap (I got mine for less than $1 a yard) to not really at all expensive for fabric (I saw some tulle for like $6 a yard - it was rather fancy looking). Two yards will suffice for a children's tutu. I doubt most of you are wanting to make one for a kid though. I used six yards for mine - my tutu is ridiculously poofy and has about a six inch gap with no material. One day, I'll fix that, but for now I just twist it to the side. Depending on how you cut your stripes and tie them on, you may need more or less fabric.
First, you're going to need to cut the tulle into strips. My tulle just happened to be twice as wide as I wanted my tutu's length to be, so I decided it would be easiest to loop the tulle around the ribbon. I cut my stripes about 4 inches wide - the width of these stripes is going to determine the poofiness, so use my tutu to judge.
After you have your strips cut, you're going to want to fasten them to a ribbon or elastic. If you use ribbon, make sure it matches the tulle - or at least compliments it. I forgot to buy ribbon from the fabric store and all I had laying around with some olive green piece. There are several methods for fastening the strips to the ribbon - you can sew them, tie them, or probably even use fabric glue. I opted to tie them on. Because my strips were twice the length I wanted, I just doubled them over and looped them around the ribbon.
Because I am such a horrible explainer of what I mean by 'looping around the ribbon', I drew a picture. Unfortunately, I am also a horrible drawer. I opted to crop out the final result, which is what you achieve by pulling the ends tight, but I don't think it's a crucial element. Just know that in the drawing below, you are actually looking at what will become the inside of the tutu's waist. You want the clean part of the loop on the outside, as in the photo above.
You may want to play around with other methods of fastening the strips to the ribbon.
If you have any questions, I will try my hardest to answer them.