First step: cut off the top, leaving 2-3 inches of fabric longer than you want the skirt to be.
Next step: cut your elastic to the correct length and sew the ends together to form a circle. Turn the skirt inside out and pull the waist through and then over top of the elastic.
Next: sew through your elastic as you work your way around the skirt. A zig zag stitch will help to reinforce it as well if you feel like the straight stitch isn't strong enough for the weight of the elastic.