The LED lights and electronics all attach to an underskirt or slip that can be worn underneath your costume dress. I like this setup because it adds so much versatility: all this hard work can pay off with an unlimited number of dresses, and it's easy to change the colors or programming to match.

I started with a long pencil skirt that I made myself out of some leftover fabric. There are a lot of simple sewing patterns available online. You can also find already-made pencil skirts on a lot of inexpensive clothing web sites. Look for one with a seam in the center back. I recommend cotton or linen, or another non-stretch, breathable fabric. My skirt is a non-breathable polyester and I really regret this choice when I'm sweating through three diffusion layers.

Sew the skirt together but leave the back seam completely open from top to bottom, to make it easier to lay out and attach the pixels. Or, if you're using a premade skirt or slip, use a seam ripper to completely separate the back seam. This will enable the skirt to lay (relatively) flat on your table so you can see all sides at once. 

Starting in the upper left corner of the skirt (the back center), start to lay out the pixels in a serpentine grid. I used a hot glue gun to temporarily attach the strands down to the skirt. This worked great while I was getting everything laid out.

You can make your grid any size. Mine ended up with 19 vertical columns and 20 rows of pixels. This is easy to change and configure in the software so let your costume vision determine the correct number of pixels.

However, you want to get as close to an evenly-spaced grid as possible. These strands have 5cm spacing in between the lights, so I placed the strips 5cm apart on the skirt, so that any four LEDs make a perfect square. 

The fabric will move and shift, so you don't need to get this 100% perfect -- it will still look amazing if you're not laser-precise -- but the more evenly spaced you can get the lights, the more options you'll have for really cool animations.

Pay attention to both the vertical and horizontal lines created by the lights. When you get to the top and bottom you'll need to pull the strands around a bit to keep the spacing even on the longer runs. Don't worry about this too much. It's the lights in the center that really matter the most so concentrate on getting those lined up as perfectly as possible.

Once you're happy with the layout, use a sewing machine with a wide zigzag stitch to permanently attach the lights to the fabric. You can also use embellishing glue or silicone glue if you aren't a sewer, but glue never holds quite as well as a good ol' needle & thread.

Make a pocket for the battery and microcontroller on the inside of the skirt. I used thick fabric with a little stretch to protect this stuff and keep it from moving around.

I added an elastic waistband with a large hook & eye closure and then sewed up the back seam, leaving the top foot or so open. You can add a zipper or closure here if you'd like.

Diffusion

This skirt looks great as a grid, but sometimes you want the lights to blend together a bit more. The best way to do this is to add a diffusion layer. I tried a few different options. My favorite is a fluffy tulle crinoline skirt layered with an overskirt. The tulle adds space and good diffusion between the lights and the costume, without adding dark bands the way a hoop skirt does. 

Crinolines come in all different lengths and shapes, so do some web searching to find just the right one.

The hoop skirt worked well too, but back-lighting the steel hoops leaves a few black shadow lines in the middle of the light field, which wasn't the look I was going for.

I layered a white ruffled skirt over the top of the crinoline and got a gorgeous diffused look.

For another look, I layered a butterfly wing cape over the crinoline, giving my dress a glowing, pulsing butterfly wing skirt. I also made a "fire" mode on the skirt using WLED's Fire2012 effect and twirled around like Katniss from Hunger Games -- this dress will do that look too. There are endless possibilities!