First, and most importantly - pick your fabric, and your top. I used old vest tops but in all honesty I can't see any reason why it cant be done with ANY fitted t-shirt. I also made one skirt that was the same colour as the top and one that was contrasting, and either works. I've just used cotton - its a bit warm for anything else at the moment!
Measure your waist and the length from your natural waist to where you want the skirt to sit. You will need two panels of fabric the width of your waist and this length, plus an inch for seam allowances. This gives you the extra fabric you need for the gathered skirt. You'll also need some 1/4 inch elastic the size of our waist + 2 inches extra for the waistband.
Sew these together to make a tube. I used the entire width of the fabric on the flamingo dress because I'm a big girl and I liked the idea of a very gathered skirt, after using less on the willow pattern one. I wouldn't recommend reducing the width of the tube in all honesty. It seems a massive amount but I think it does look better.
Once you have your tube, baste two lines of stitching along the "top" of your skirt, seem to seem and on both sides. Knot the ends but leave a good length of threads on either end. Pull these until you have an evenly gathered that matches the size of your waist.
Now put on your shirt (inside out it best for this) and mark the line of your natural waist, then cut about 1/4 inch below it. I used my rotary cutter to ensure a straight line.
Once that it done, right sides together, pin the skirt to the top, keeping the gathering even. As you can see, a husband is handy for this! Once you are satisfied that it is properly pinned, pin the elastic in place.
And there you have it, that easy. If you make your own please post links to photos in the comments. I'd love to see them!