you can get some elastic for your waist and tie a knot in it.

cut strips of tulle double the length you want.

tie the strips of tulle on to the elastic with the knot (where you tie) in the middle of the tulle. each strip will be 2 layers of the skirt.

that's it, there are plenty of tutorials if you want pics to show you.

* softer tulle will be easier to work with
* use more than one colour tulle to make it pretty
* if you can get a roll of tulle (15cm wide), you won't have to cut the width of the strip, just the length.