One of the Holy Grails of LED costume design is the quest for the Stretchy Circuit.  

Clothing needs to move and stretch with the body.  Wires and electronics do not stretch, as a rule.  

For this project, we're going to use LOTS OF WIRE.  We'll zig zag the wire so we can stretch the fabric without breaking it. Since we're making an under-layer, we can get away with using an abundance of wire. The main costume layer should hide the wire nicely while just letting the light shine through.

Denier Tricot is a wonderful fabric for slips and under-clothing.  It's made for lingerie production so it feels nice next to the skin.  It has a 2 way stretch, meaning it'll hug your body and stretch left-to-right, but won't stretch in length, even if you sew lots of heavy wires and batteries to it.

I made my own since I had some Tricot on-hand; you can do the same or order one pre-made from Amazon or your favorite lingerie supplier.  (It might be fun to use a tulle petticoat or hoop skirt as well!)

Layout

Now it's time to figure out how many lights you want to add and where they will be placed.  

My skirt will have two LED strands with 10 neopixels apiece, mirrored on the left and right sides.   Since I'm mirroring, I only need to map one strand, then I can copy it on the other side. 

NOTE: I finished my whole project using this diagram, then later realized I may want to wear this under a dress as well as under a skirt.  If I were to do it again, I'd add an extra 12-18 inches of wire between the Circuit Playground and the first neopixel in the strand.  That way I could wear the Circuit Playground module as a belt buckle or as a brooch, clipped to my neckline.

Mark the Pixels

Lay the skirt out on the floor and mark where you'd like each LED to be.  Draw a line connecting all the pixels, with arrows showing the flow of data.  Connect + and – from the first pixel to the last pixel, making a rough circle (do not connect the data wire back to the first, only the power wires).

Find Wire Lengths

Measure the distance between each planned neopixel spot.  Multiply each measurement by 1.5 to allow for stretch and movement in the wires.  Note down all the wire lengths.   Number each measurement, so you don't lose your place later on.