Summarize this article:

After you have inserted the elastic all the way through the fabric, you will need to sew the fabric and the ends of the elastic together to close the pieces. This will allow the elastic to gather the shoulder and dress pieces at the top of your dress. Sew the fabric and the ends of the elastic piece together leaving a ½” seam allowance. Make sure to sew the fabric on the wrong side so that when you turn the pieces inside out, the seam allowance will be hidden. After you have closed up the back of the shoulder and dress pieces, you will need to hem the bottom of the pieces. Fold and pin along the bottom edges of the shoulder and dress pieces to create a ½” seam. Next, line up the back seams of your shoulder piece and dress piece and pin the pieces together at the elastic in the front and back. When you are happy with the positioning of the pieces, stitch a line down the front and back of the elastic at the very center of the front and back to secure the pieces together. You can simply stitch up and down the width of the elastic pieces two or three times to ensure that they are secured.  Use a thread color that will blend in well with your fabric. This will help to ensure that the stitching is not as noticeable. Make sure that there is an even amount of shoulder piece fabric on either side of the dress piece. Otherwise, your dress may end up looking lopsided. Make sure to sew through both layers of the elastic and fabric. After you have sewn the shoulder piece and the dress piece together, your dress is ready to wear! Try on your new dress and see how it looks.

Summary:
Sew the elastic and fabric together. Hem the bottom edges. Secure the shoulder piece to the dress piece. Try on your dress.