I cut a piece of elastic to fit her little waist and left a bit of room to secure it when I was finished. I placed two safety pins through each end and pinned it to the back of the couch. This was such a huge help for keeping it in place and making it nice and easy to knot the tulle to. I really have no idea how else you would do it, so this method kept it in place and kept my comfy on the couch while making it.
I started by folding the tulle in half, and half, and half, and so-on until I had a piece that I thought would be a good size. I kept in mind that, with the way I would be knotting it over the elastic, I would need them twice as long as I wanted the tutu to be. By using a slip knot in the middle, you end up with two long, equal strands hanging down.
To make the slip knot and attach the tulle to the elastic, I folded the tulle piece in half, brought the folded center over the top of the elastic and down around the back. I then pulled the two ends down through the loop that created. It helped to keep the knot neat if I used my fingers to hold it rather than pulling agains the elastic (that will make more sense as you find yourself doing it).
There are two different looks the each side if the knot. One side shows the piece the loops around, but the other side looks a bit neater with just the two ends hanging down. I like this for the outside because it looks a little fuller.
I originally cut my pieces too long, and about halfway through I realized they needed to be smaller to be as full as I wanted. I cut them in half and the tutu ended up about a foot long and sticks almost straight out. Totally cute for this costume, but this is why I recommend way more tulle for older children or a tutu that was not being worn over a costume.
I chose to make mine a no-sew project and just knot the elastic together in the back. This is only a Halloween costume, and the fullness hides it pretty well, but it would be about the easiest thing in the world to just zip the sewing machine over it a couple times.