First and foremost, you'll need a corset. This method of attaching LEDs is designed to work with a white or light-colored corset, one that will reflect the lights and appear to glow on its own. A shiny satin fabric in white or silver will work best.
There are numerous online tutorials on making your own corset, or you can find inexpensive to high-end bespoke corsets online. I've tried quite a few different vendors. Here are my favorites:
Inexpensive Costume Corsets
I've found these everywhere from Halloween stores to Etsy to Amazon. The fit is never perfect, the structure is never ideal, but if you're not trying to alter your shape drastically you can get a nice looking corset for around $35.
Corset Story is my favorite vendor for pre-made corsets. Their corsets are in and around the $100 range, and the construction is solid. I used their Classic Long White Waist Taming Corset in this tutorial. I've also ordered from them for several other costumes (including my animatronic Medusa costume) and been very happy with the quality and fit every time.
High-End and Bespoke Corsets
My preferred vendor for high-end corsets is Dark Garden Corsetry. They're based in San Francisco and New Orleans, and specialize in custom work. For the most comfortable fit imaginable in a corset, combined with superior construction and unbelievable shaping, dig into your savings account and get in line and these lovely ladies will create something unique for you that will last the rest of your life.
I placed one strip of pixels along each boning strut on my corset. My corset has 11 struts total (12 if you count both sides of the front closure). This worked out to just short of 5 meters of pixels total.
I could have possibly gotten away with 4 meters, except that these pixels are difficult to re-connect once they've been cut because of the tiny size of the solder pads. I recommend getting enough pixels that you have extra and don't need to "frankenstein" any of the strips together in the middle. You can always use the leftover bits for accessories - maybe a glowing pocket square or a top hat for your date.
I used black ribbon to cover the NeoPixel strips because I love the black-and-white striped look, but you could use white ribbon for a magical simplicity, or red ribbon for a Mary Poppins look.
Selecting your NeoPixels
Adafruit's Side-light NeoPixels are really great. Instead of pixels on the face of the strip, the pixels are on one side, which gives you instant indirect lighting. The strip still flexes along its length, so these are really perfect for attaching to the outside of a costume. They'll flex along your waistline and will shine out onto the fabric instead of into the eyes of whoever's looking at you. You don't need to muck about with redirecting the lights or diffusion. They basically diffuse themselves.
You can affix the strips either face up or face down, and the visual effect is the same. The handy thing about this fact is that we can wire them tidily end-to-end and affix them to the corset in a serpentine pattern, and still control which way the lights shine by alternating which way we place the strip - face up or face down.
This is a bit hard to describe, so here's a graphic. If it still doesn't make sense, it will when you start laying out the corset.
The strips come in 3 flavors: 120 lights per meter, 90/m, or 60/m. There are advantages and disadvantages to each type.
The 120/m ones look simply stunning. The animation is smooth and the light effect is really robust and complete. They're jaw-dropping. But they do take more battery life and more processing power, since you'll use a lot more pixels at 120/m. They also cost a bit more.
The 60/m lights are kind of sparse. The battery will last twice as long, but the lights are far enough apart that I definitely notice a difference -- it looks a little bit spotty. Fine if that's what you're going for, but I found these too sparse.
The 90/m lights are a good compromise between total number of LEDs and LED spacing. They don't look all that different from the 120/m lights, at the end of the day -- the diffusion makes the difference barely noticeable. However, because of the way the strips are laid out (they're narrower with smaller soldering pads) these are devilishly tricky to solder. If you're a master solderer, that may not be a concern, but .. I've been doing this LED costume thing for a while and still had a really tough time.
Oddly enough, the 120/m ones were a snap to solder compared to the 90s (unlike most high-density strips).
A Good Compromise
I ended up using half 120/m pixels and half 90/m pixels. I put the 120s in the front of the corset, then switched to 90s for the sides and back. This kept my total number of pixels down to around 650 pixels, which is easily handled by the Circuit Playground Express. With the pixels at a decent brightness, my battery lasts over 4 hours. And I only had to solder half as many devilish 90/m strips.