I have put all the steps below, obviously this is a one-off but I hope this tutorial inspires you to change some of your favourite pieces.
Step 1. First open the seam that attaches the lining to the main skirt, you need to get inside to see what's happening
Step 2. Cut the front layer along the seam line, you will now be left with four layers of fabric, this is good as it with form a waistband
Step 3. Try it on check all edges are going to line up.
Step 4. Cut off the excess zip teeth but don't let the zipper come off
Step 5. Pin and stitch bias binding along the raw seam edge
Step 6. Flip over and stitch the front of the bias binding to finish the edge.
Step 7. Fold and stitch the end if the bias binding back on itself to neaten and the back stitch across the top of your zip to secure it.
Step 8. Hand stitch the newly formed waistband to the lining.
Step 9. Make any other amendments, I had to add a patch to cover a tear on the front.