I used a pair of shorts that were really too short to be decent, and a vintage bedsheet. (It was originally purchased for the quilt I made for my daughter last summer, but there was still over a yard left.)
The bottom tier is twice as big as the middle tier...which is twice as big as the circumference of the shorts. The entire length of the skirt is 11" (the bottom tier was cut 6" tall and the middle tier was cut 5" tall).
I sliced off the leg portion of the shorts just above the crotch, which took about an inch off the shorts length (seriously short shorts!) and attached the skirt to the raw edge.
In hindsight, I'd like to try this again, attaching the skirt to the waistband of the shorts and leaving the shorts intact. That way, she could flip and tumble and nary a Hanes to be soon. (Woot for that!) Maybe with a pair of jersey shorts...
(This was inspired by a post at CraftEvolution)