Here is what I do to make the tutus. I originally found some online tutorial for it, but I can't find it so I am going to type up what I do because some of the ones I am finding now are not as easy as what I do.
I order my tulle online from PaperMart.com
I buy it in the 6" spools. I find that easier, but you can buy the bolts and cut the tulle to 6" wide pieces - cut the entire length of the tulle. I generally try to put together 2-3 colors, but even just one will look great. Naomi's tutu dress took almost 5 rolls so approximately 150 yards and Gwen's only used 3 rolls about 75 yards. I wanted Naomi's to be really poufy and Gwen's to lay a little flatter.
All you need next is elastic and ribbon, if you want it. I buy elastic that is about 1" wide. I don't know if the width really matters though.
I first measure how long I want the tutu to be. So if you want a dress measure from under the arm to wherever you'd like the bottom to lay. Then you cut the tulle to be double this length in strips. So gwen's dress is about 21" long so I cut it to 42". I like to find something that is close to that size so I can wrap the tulle around and around it and cut once. For these dresses I wrapped it around a small Ikea table that was 21" across last time I cut cardboard to the size I wanted. You wind up with a lot of 6" x 42" strips. You can do skirts as short as 10" which would be strips cut 6" x 20".
Then you measure the elastic around wherever you want it to be placed. I have read that if you take off 2 inches from this measurement that should work. That hasn't worked for me yet. When you tie the tulle onto the elastic it stretches out so I just tie the elastic and work with that so I can adjust when I get closer to being done. This time I just left it tied and put tulle over the knot, but last time I sewed the ends together once I found the right size.
I pull the elastic up over my leg up around my thigh so its at a comfortable working level. Then I just grab pieces of tulle and I tie them around the elastic like this:
I pulled the knot really tight for Naomi's which is what made hers so poufy. There is a lot of tulle wrapped around that elastic!! Gwen's was not pulled as tight. You can do every other color, but i like to do 2 strips of one color and then 1 of another. You can vary it up. I think it gives the colors dimension instead of just striped. Gwen's butterfly tutu goes 2 brown, 1 orange, 1 cream, 1 orange, 2 browns.
Once you get all of the tulle you want onto the elastic then you can try it on for size. With my girls I always have to undo the knot and re-tie it tighter, usually a lot tighter. The tighter your knots are the more the elastic stretches which also means it will stretch less overall once its on so you do need to leave some stretchability in the elastic. Like I said at this point you can probably sew up where you had the knot, but I didn't this time. Then you finish it off with a little more tulle tied around where you sewed or where the knot is to cover it up.
You can tie ribbon around the elastic to have a nice bow in the front or back. If you did a dress just loop the ribbon around some of the tulle and up over the shoulders to hold the dress up. Then I just tie ribbon around their waist. You don't have to do that, they could just run around in a pouf of tulle!