ribbon or elastic
a crap ton of tulle
(its easiest to use a 6in wide roll and cut the length to your specifications)
scissors or blade
Ok, so you choose the length you want your tutu to be (usually 10-15 inches) and then double that. It depends on how tall you are and what look you're going for, the shorter length will be more of a classic sticking way out but not really covering your butt style; longer will hang differently and give you a very different look.
Once you have your strips, you attach them by looping them over the ribbon or elastic. If you pinch the end with one hand while pulling to the top with the other, it helps to keep the length even throughout the process. You can either tie or loop the ribbon or elastic over your leg or onto something like a chair back.
Some folks suggested using elastic and sewing the ends together, others suggest one of those elastic headbands. What I found while doing this is that I actually prefer just using ribbon. Because you end up stretching the elastic so much while making that puppy, by the time its finished its all stretched out. During this one I actually snapped the elastic during an early stage, which meant I had to start over. Not cool.
Ditto with the hair elastic bit. It gets so stretched out as you go that its super stretched out by the time its done.
For my second one, which I'll show another time; I used ribbon instead. I put the ribbon around my waist and figured on some extra length to tie it in a bow. Then I just tied a knot on each end at the right place. That way I can just tie it on, super easy and tying it with a big bow looks lovely. It makes little difference what kind of ribbon you use, although a thick grosgrain ribbon with make for a thicker waistband and will also tie into a loverly bow at the back. ;)