You will need...
-Elastic the length of your waist in inches plus 3 inches
-A rectangle of fabric, double your waist plus 5 inches for the length. For the width of your rectangle, measure from your waist to the top of your knee
First, at the bottom of your rectangle, fold over twice and iron the hem flat. Sew the hem line in a straight stitch.
At the top of your skirt, eyeball a pocket for the elastic. Fold down and sew the pocket just a little bit wider than your elastic.
Feed the elastic through the pocket using a safety pin.
Straight stitch up the side and you're done!