You will need:
Start with the dress that you want to turn into a costume. This dress was gifted to me by a friend, it was too short but the fabric was lovely. I decided to turn it into a practice set. If the dress has a zipper, carefully remove it with a seam ripper. Mark a line app. 1 inch below the bottom of your bra and cut the dress in two.
Fold the lower edge of the top part in (app. 2 cm/ 3/5 inch) and stitch a tunnel by stitching 1,5 cm/ 3/5th inch from the bottom. Thread elastic through tunnel, then close the tunnel. If the dress had a zipper, remove it first. Instead of threading elastic through the tunnel, the
top will probably already fit without pulling up when you raise your arms. Add buttons to the back or put a new (shorter) zipper in the top.
Take the skirt and measure whether it’s too short, long enough or too long. If it’s too short, add a ruffle to the bottom of fabric that matches the dress.
If the skirt is too long, pull the skirt up for the desired length and mark this on the top (where the waistband is going to be). Make a waistband out of matching fabric (broadest point of your hips, add 2.5 cm/1 inch for seams on sides). Pin skirt to waistband, folding
and pleating extra fabric. Then fold waistband over and stitch together on bottom. Thread
elastic through waistband, then close tunnel. If skirt had a zipper: Lengthen/shorten
according to steps above. If skirt is too big, close the seam where the zipper used to be and attach a waistband. If skirt fits perfect with zipper, put in a new (shorter) zipper and a
hook and eye closure on the top.