Back curve 1
Left in the picture: Cut two pieces of strap elastic. Each piece of elastic should be about 8cm (3″) long. Line up the elastic so the cut edge of the elastic is even with the center back of the wing, and the upper edge of the elastic is even with the back curve of the wing. Using a narrow zig-zag, stitch the strap elastic in place first along the outer edge of the elastic, then again at the inside edge (1). You should have about 3cm (1.25″) of extra elastic at the upper edge of the wing.
Right in the picture: Thread the excess elastic through a ring and fold the elastic over the ring. Stitch in place using a straight stitch or very narrow zig-zag stitch (2). Trim the excess close to the stitching.
Back curve 2
Position the bra and elastic strap so the right side of the bra and wrong/plush side of the elastic are facing upward. Now thread the free end of the elastic strap though the ring sewn to the back wing (2), then back up through the slide adjuster (3).
Sew the free end of the elastic strap to the edge of the cup using a straight or narrow zig-zag stitch.
Cut two 30cm pieces of the bra strap elastic. Attach a ring to the fabric part of the strap and a slide to the elastic.
Thread the other end of the strap elastic through the ring attached to fabric strap and then back through the slide adjuster.
Finish the back curve of the band with the free end of the elastic. Attach the strap to the cup as well.
Attaching the strap to the cup
Attach the strap to the edge of the cup using a narrow zigzag stitch. Trim any excess fabric strap close to the stitching.
The Back Closure
Open the hook closure. Position it so the foldline of the hook closure is even with the center back edge of the right wing. Sew in place using a zigzag stitch. Turn the bra around, fold the tape to the wrong side of the bra and sew in place with straight stitch. It helps to have the needle positioned as far right as possible. Using a zipper foot helps as well.
Reinforce the ends of the casing
To prevent the underwires and optional boning from popping out, reinforce the upper edge of the underwire and boning casing with a bartack (very narrow zigzag stitch).