Plug your NeoPixels into the NeoPixel port on the back of the Hallowing.
Then, plug your extension cable / switch into the 2-pin power port on the Hallowing. Plug the battery into the other end of the switch cable.
You'll get slighlty better results if you assemble the mask while it's curved into its final shape against your face. I used a foam wig head to hold the vinyl in place while I added the LEDs.
Place the Hallowing on the mask so the TFT screen will show through the window. Secure it in place with hot glue.
Find the center of your pixel strip and place that right at the top of your Hallowing, centering the pixel strip on the mask. Use a needle and thread to stitch the NeoPixels around the outer edge of the mask so the lights are facing inwards.
Use your needle and thread to secure the wires from the Hallowing to the NeoPixels, so they're hidden as much as possible.
Use silicone glue to glue the front of the mask to the NeoPixel strip. The silicone sleeve on the NeoPixels protects them very well, but only silicone glue will stick to it! Hot glue or E6000 will not work here. Silicone is slippery stuff.
For the Dragon mask, mount the Hallowing upside-down at the bottom of the mask so the flame image looks as though your dragon mask is breathing fire.
Poke a hole next to the eyes of the mask and thread through an elastic band to tie it to your head. Add a rubber band to the battery and use a hair clip to hide the battery and switch in your hair or on your collar.
Optional: add a 1"-1.5" cabochon over the TFT screen to give the animation some dimension and shape. Glue on ribbons, jewels, edging, or anything else you'd like to make your mask special.