The tulle I used was actually used as the chair tie backs for my little sister's wedding, so it was free for me :). I always prefer to just use what I already have or can get for free (or close to it). So if you don't have a sister with tons of tulle, you can get this kind of tulle on spools at most any craft store. The tulle I was given was pink, purple, blue, and yellow, but you can use whatever color you want. I am not sure how many spools of tulle you will need, but I think you will want at least 6. It will kinda depend on how full you make the skirt. If you know you are going to want it really full you might want 8 spools.
Once you have your tulle I cut it the length of my arm. So just look at the bed you want it on, pull a length, fold it in half and get an idea for how long you want to cut your strips. When you are cutting keep in mind that you will be folding these strips in half and cut them about 4 or 5 at a time.
Once they were all cut I set my sewing machine on regular tension with a long stitch length. The I sewed about 3 strips at a time. So I would just pick up a pink and fold it in half, then a blue fold it in half, and then a pink again and fold it in half. Then I would run them through the machine. I used a pink, blue, pink, yellow, pink, purple pattern.
Measure around then bed you are making this for to know how long to make it, Once mine was long enough I just sewed it onto her existing bed skirt (because I liked the idea of the light pink of the existing bed skirt backing the tulle.)
And there you have it.