Cut a strip off of your big rectangle, as wide as you want your waistband to be (plus seam allowance.) Part of this will become your straps, but they'll be thinner because you'll double them over.

Hem both sides of the big rectangle.

Hem your waistband.

Attach your waistband to your large rectangle by pleating it evenly. (Click the link in the description for a more detailed description and picture.)

Sew over your pins. Sew the skirt closed, too.

Fold your straps, one at a time, right sides together and sew, then turn right side out. Sew them on to your dress and you're done! Now spin around! Wheee!