First, the basic idea: You're going to cut the shirt straight across from underarm to underarm and flip it upside-down. If you started with a shirt that fit your waist well, you're done already. Seriously! I like to leave the bottom unhemmed if I'm working with jersey fabric since there's no worry about fraying. If your shirt was a little big, you may want to take in the sides a bit, just to make sure you don't have a wardrobe malfunction.
Or you could add a bit of a feminine flare with ruffles. Cut "C" shapes out of the remaining fabric of the collar and sleeves. Then straighten out the C and sew its inner edge to the bottom of the skirt.
Another option would be the addition of a fabric remnant from your local craft store to create your own maxi skirt. In my case, the white shirt I used was far too thin to stand alone, but worked quite well as an underskirt for the also-too-thin fabric remnant. I also decided to insert a length of elastic in the waistband this time, instead of relying purely on the elasticity of the shirt fabric to hold up the weight of the skirt. I just cut a slit on the inside of the waistband, threaded the elastic through, and stitched the slit back closed. You could even make yourself a matching headband if you trim a strip off the fabric remnant.
Come back soon, and I'll show you what I did with the top of that white tee!