For the exposed casing, cut a strip 3″ wide. I made mine much longer than needed and trimmed it later. Press over 1/2″ on each long side.
Lay it on the dress and pin it down. You may want to try on the dress and mark your waist before pinning the casing on. Make sure it is nice and straight as you pin it all the way around. Start at the side seam.
Thread elastic through using a safety pin. You’ll want the elastic to be slightly smaller than your natural waist. Sew it together.
Trim the casing edge and fold over. Sew it up.
I took in the sleeves by simply sewing from the armpit to the end of the sleeve, taking in about 1/2″ the whole way.