Flip the leggings right side out and sew around the edge of the elastic using a twin needle. Using a twin needle will give you the stretch you need around the waist and hem. Slightly stretch the elastic as you sew and be sure you are catching the edge of it. Hem the bottom of the leggings while also using a twin needle. If you don’t have a twin needle a zig zag stitch or any stretch stitch will work ad well.