A few dollars' worth of tulle and a couple hours' work were enough to provide me this very simple tutu. Most of the online instructions seem to be for making costumes for little girls who want to be fairy princesses or something - the steps are the same, but the colors are likely to be different for derby. So here's how I did it; and if making a tutu seems like too much trouble, my friends at Sue Zq Bows will be happy to fix you up with a custom tutu of your own.
For a tutu, you'll need tulle in one or more colors - more than you think you'll need. I used about 6 1/2 yards of black, and 4 1/2 yards of green. If you're not a regular fabric shopper, know that this stuff often comes on sale, and that fabric stores usually run 40% off coupons, so never pay full price for your fabric. I've read you can get tulle in long strips in the wedding section of craft stores - that would be easier to work with, but you won't have as many colors available.
Cut the tulle into strips. It's much easier to do this neatly with a rotary cutter than with scissors. I made my strips 10" wide. Then cut the strips to be somewhat longer than twice the length you want the tutu to be. I decided that my tutu would be about as long as 1/4 the width of my tulle, so I could get two strips from every 10" of fabric.
Wrap a piece of no-roll elastic, about 1" wide, around your waist where you'll wear the tutu. Measure over the sort of clothes you'll wear the tutu over. Stretch the elastic just slightly as you wrap and cut it, then overlap the ends about an inch and sew them together into a loop.
Take a strip of tulle and fold it in half. I offset my ends a bit to try to get a little more length in the tutu, but you can also fold them evenly.
Lay the center of the strip of tulle under the elastic. Bring the ends of the tulle up around the elastic, and pass them through the loop formed by the center of the strip, making a lark's head knot.
Towards the end, you may need to stretch the elastic slightly or push the knots together. You'll want the elastic to be completely covered, but you don't want to cram on so many strips that the tulle stands out straight. You'll know when your tutu is finished. Once it's finished, you can embellish it with ribbons or flowers or sequins to your heart's content. Or make another one - it's that easy!