Measure and cut the waistband
Cut the skirt, make sure it goes around your waist at least one and a half times; two to three times is better if you want it more gathered :)
Set your sewing machine to the loosest stitch and sew one or two lines of stitching along the length of the fabric, gather it.
Now pin the skirt to the waistband and sew,
Pin and sew the zipper, and hem the skirt.
and viola! you're done ;) <3