This actually uses quite a bit of math!
1. Measure your waist--this is the circumfrence.
2. Using the formula C=2(pi)(r), solve for the radius.
3. Fold the fabric in half again (after the fold it comes with from the store). In the corner where all edges are folded, mark a quarter circle with that radius.
4. Whatever you want the length to be, mark a larger quarter circle in addition the radius. (If the skirt is supposed to be 20" long, add 20" to the radius--don't start at the corner.)
5. It's probably wise to give your self a bit of extra room at the top so you can get it on.
6. Whatever your circumfrence is, make a waistband of the same length (plus a little bit extra for the seam). This is where the elastic goes.
7. That's pretty much it, besides the hem.
Hint: It's probably best to use 60 in wide fabric, otherwise, you'll end up with a 10 in long skirt or something.
Easy, cute, and twirly!