For the Tutu I ended up making it around an elastic and doing a square knot, just like a regular tutu, but I only did the front. I put the elastic around the waist of the bodice and reinforced with glue. I used ribbon to cover the knots at the waist of the tutu. I posted pictures here
. The bottom was even because I used the same length tulle in each knot. Or you could trim it after you are done if it doesn't look even.