Hemming Formal Dresses Doesn't Have to be Hard! Nancy's Notions I was wearing my junior prom dress. It was black, without straps or sleeves, extremely poofy on the bottom, and had a U cut on the ends so you could see my How to Alter Prom Dresses Naomi Fata Easy way to hem a prom dress. Its easier than cutting first. You sew first. , mark the hem. Subtract 1 2 for hem. , mark with chalk marker and turn to the inside on chalk marks. I hemmed 2