Insert one end of the elastic in the top, start sewing, catching the elastic.
After the elastic is caught, form 2 pleats in the middle of this side, that you will sew in place as you sew the side shut.
Insert other end of elastic into place BEFORE you finish the side. Don't forget to Backstitch.
Pin the pleats down on the other side so that the pleats line up, and sew the second side inserting the elastic as you go.