Your costume idea is wild, exciting, and fully formed in your head. Here are a few things to contemplate before you turn it into reality.
Whoa, there, Pardner
Decide how many lights you want in your costume. Then cut that number in half, and see if you can get the same effect. Fewer lights will mean longer battery life, and using more than around 300 can start to become unmanageable. Do some sketches and see how few you can get away with. One nice thing about these dot strands is that they can really cover a large area with just a few lights, and if you're artful with placement you'll still get that full-body light effect with a tiny battery.
You can always add more strands later on if needed, but starting small will give you a much higher chance of success.
Check out this Sipping Power with NeoPixels guide for more info on how to get the most battery life out of your costume.
Wiring in Parallel vs Wiring in Series
I'm wiring all my LED strands in parallel. This means I've got the strands in a starfish-type configuration, where each strand is directly connected to my microcontroller's data pin. My microcontroller sees only 20 pixels, even though there are 200 total. Every strand will show the same animation on its 20 pixels, mirroring all the other strips.
If I were wiring in series, I'd connect the strands end-to-end, wiring the OUT end of one strand to the IN end of the next strand. My microcontroller would see 200 pixels and be able to run animations that wander from one strip to the next.
Benefits & Drawbacks
If I wire all 200 of my LEDs in series, I can run more complex animations. For example, I can do colorwipes that fade from one end of my costume to the other, or gradients that slowly change along the full 200 light layout.
However, if my power, ground, or data wire breaks or gets corrupted at any point, or if any one of the lights breaks or goes bad, every single light "downstream" of the broken place will stop working. NeoPixels need a solid power, ground, and data signal to light up, and if any of that gets interrupted, my lights go out.
Wiring in parallel is a bit of a trade-off: I can only write to 20 LEDs, so I'm a bit more limited on whole-costume colorwipes or gradients.. but if any one of my lights breaks, the maximum number of lights that suddenly don't work is limited to 20.
By wiring in parallel my costume fails gracefully: when one of my lights breaks, part of that strand will stop working, but the other 180 lights will continue to work just fine.
In the sketch above, the yellow boxes show LED #1 on each of my strands, and the red arrows show data flow direction.
LED Strand Positioning
Figure out how you want to position your strands in order to minimize extra wire running through your costume. Extra extension wires love to get caught on things and pull out. Put the IN ends of your light strands in the same spot, if possible.
Data Flow Direction Layout
Also, think about data flow direction. In this design, my patterns and animations will radiate from the center of the tail and flow outwards. This will give me a gorgeous symmetrical look with a lot of motion.
Your costume needs to move and bend. These wires are really flexible and can bend a lot of times without breaking, but elbows and knee joints want to flex constantly. Some costumes require some pulling and stretching to get in and out. Think about all these weak points and do not put lights there. Seriously. A dark spot can be artful if it's planned, and the lights will eventually break at all your flex points. I promise.
My mermaid tail has a heavy silicone fin inside and when I'm maneuvering on land, I need to stand up in it. I left the entire back-of-the-heel area completely unlit, since it's unwise to stand on my LED light strand.
Mounting in your Costume
Think about how you plan to attach the lights to your costume. You may want to wash your costume, someday, and sewing the lights directly in can make this more difficult.
You will also need to be able to get to the lights easily when a strand breaks, or if you want to reposition the lights or add more. If the lights are potted in glue and silicone and embedded permanently into the costume, repairing them will be a nightmare.
I attached my lights to an underlayer / lining that I made out of scuba fabric. It's got about the same amount of stretch as the fabric tail I'm using. It's worn under the tail, but not attached to it in any way, so I can still wear my tail without lights for daytime shows, and I can also wear the lights with different tail skins.
I patterned the lining from a Mertailor Whimsy 3 tail. The lighter color designs will show the lights a bit better than the darker ones.
I used my sewing machine to stitch my lights in place using a wide bar tack stitch.
This was fairly quick to do, but ended up causing a short when I took it into the water. I was careful as I could be not to pierce the wires with my sewing needle, but I must have hit the wires somewhere and made a hole because the strand flickered when it got wet.
My solution was to cover each bar tack with clear nail polish, to seal any potential holes in the wire. If I were to do it again, I'd use fabric glue to attach the lights instead.