Attach the plastic bottle of the fiber optic puff with Velcro dots to the wrong side of the top costume body. Pull the black and red wires of the fiber optic puff through the lining buttonhole. Connect each wire to the wires of AAA battery holder, using the same crimp-on connectors as you did in step 4. Make sure connect black to black and red to red.
With the wrong sides together, add the elastic band to the head piece and edge stitch the top and lining together. Try it on the dog and sew on your Velcro.