Serge ONE of the side seams, matchgin the binding seams. Leave the other side open




Measure and apply elastic. I use soft waistband elastic starting from one of the open edges serge elastic to wrong side of top edge, pulling as you go. Again, marking quarters and/or pinning would help, but I'm lazy



With elastic fold inwards sew the other side seam. Make sure to match the elastc and binding in both sides