Step 5: Stitch the casing down.
Step 6: Insert the elastic into the front and back casings using your favorite method. I use my trusty safety pin. Be sure to pin the elastic in place at the edge before it disappears into the casing. Here, you can see how the elastic gathers the back and front edges.
Step 7: Finish armhole edges with bias tape. First, pin the bias tape to the armhole edge, right sides together, leaving 1/2" hanging over on both ends. Before beginning stitching, fold the bias tape over the top edge snugly so that there is no gap and stitch down (as shown in the photo to the right). Continue sewing bias tape down along armhole edge until you approach the other end. Pause your sewing briefly to fold the bias tape over the end in the same manner as you did in the beginning and then finish stitching. This creates a nice, finished edge at the top edges of the dress.
Step 8: Flip the bias tape over and press the seam flat. Wrap the bias tape around to the inside of the dress, first pressing, then pinning in place. The photo to the right shows how the bias tape folds over the raw edge of the top of the dress, encasing it.
Step 9: Stitch the bias tape down close to the edge. At the top edges, make sure the ends of the bias tape are enclosed in the seam. DO NOT SEW ACROSS THE TOP EDGE. When the bias tape is sewn down, it creates the casing for the ribbon ties.
Finishing: Thread the ribbon through the armhole casings. Make sure the ends are even and tack the ribbon in place at the sideseams with just a few stitches so that they are permanently attached. Tie bows and trim ribbon ends to desired length. Be sure to cut the ends of the ribbons at an angle to reduce fraying.