Use hot glue to fix a NeoPixel dot inside each scale. It's helpful to put a little thought into layout beforehand, to be sure your lights will reach each scale. Your wires may go outside the grooves a bit on the corners, but I've found this doesn't matter too much with the finished product.
You can cut any remaining unused NeoPixels off the end of your strand. You can also solder in some extra wire between pixels if needed to reach all the scales on your design.
My Mermaid Scale necklace design also includes a printable enclosure for the Gemma, battery and switch. I didn't use this for the bra -- I just glued the scales to the outside of a bra and tucked the Gemma and battery inside the cups. But for a necklace, I want a tidier solution since I don't want the electronics to show.
I cut off the ugly, scratchy connector, and soldered three white wires to the IN end of my NeoPixel strand, and glued them to the back of the necklace. The wires measure just the right length to go halfway around my neck to reach the enclosure pod, which will sit under my hair at the back of my neck.
It's important to keep straight which wire is which! Mark or carefully trace the wires before soldering to the Gemma. If you mix them up, the lights won't work.
I cut three more white wires the same length for the second side of the necklace chain, and added a necklace closure to the end. You can find these at any craft or beading store. I added a zip tie at the other end, and glued them into one of the wire slots on the bottom of the case.
Hot glue the power switch into the slot at the top of the enclosure pod, being sure not to get any glue in the switch mechanism.
Add another zip tie to the three wires coming from the Gemma. This will serve as strain relief and keep the weight of the necklace from pulling the wires out through the case. Plug the battery in and nestle the Gemma down on inside the case.
Tuck the battery inside the case on top of the Gemma. I also cut a piece of felt as a backing to the necklace so it's more comfortable to wear, and glued the other side of the necklace closure onto the felt.
Bend the backing of the case to slip it inside the rails on both sides. It might take some adjusting to get the wires to be just the right length so the necklace hangs right where you want it.