A body form is a must for this project! You can make your own if necessary, but you really shouldn't attempt this project without one.
Suit up your bodyform with your shapewear undergarment and mock up the layout of circuitry using pins and a marking pen or pencil.
We'll be both soldering and sewing to GEMMA, so let's do the soldering first. Attach two silicone-coated wires to Vout and GND, and one to D1. Then connect one set up to Power, Ground, and Data Input on the 16-NeoPixel ring, respectively.
Solder another wire to the NeoPixel ring's Data Out pin.
When you're happy with the layout, stitch GEMMA to the undergarmend with plain cotton thread around one of the unused pins. Be careful not to accidentlaly stitch it to your body form! You may need to pull the stretchy fabric away from the form to be sure the needle doesn't pierce any fabric cover that might be on your body form.
Before affixing the ring to the garment, start stitching connections from Vout and GND on GEMMA to head to your sewable pixels. You don't have to complete the buses now, this is just so you don't have to sew uncomfortably underneath the NeoPixel ring, so you can leave long thread tails hanging out for now.
Secure the NeoPixel ring to the garment with plain thread.
Attach the Data Out wire to the garment by stripping a long section of the wire and stitching over and around it with conductive thread. Double the wire over to make a loop, make sure that it can't pull loose from the thread connection!
Since we want the right and left pixel chains to perform the same animation, we'll just split the data connection and send it to both at the same time-- they'll act "mirror image" this way without extra code! Stitch the data line to the inputs on the first two sewable pixels.
Use those extra power and ground wires to criss cross and transition to conductive thread for the far power and ground buses on the pixel chains (as pictured).
Pick up your power and ground threads and stitch on the rest of your pixels. Pull the fabric from the body form and carefully pick up bits of material to create a running stitch on your needle, then pull through. Double check you're not sewing to your form.
Gently stretch the fabric as you sew, to make sure there is enough slack in the thread to accomodate for the stretching necessary to put the garment on. Stainless steel conductive thread is very strong and will not stretch-- it will instead tear through your delicate knit fabric and could ruin your project... we've said it before but it's worth mentioning again: this project requires challenging and technical execution, if it's your first foray into conductive thread we strongly recommend you try something on woven (not stretchy) fabric first!
At each pixel, wrap tightly around the connection pad, then tie one knot and continue on without breaking the thread. Since the running stitch should be pretty loose (to allow for more stretch), this knot prevents the loops around the pixel from opening up and possibly shorting other components on the pixel.
Tidy stitching is the name of the game here! Here's a sewn pixel closeup.
It's hard to test your circuit until you've finished stitching! Once you've sewn your pixel chains, you can leave the excess power and ground threads attached in case you need to double over your connections. Just coil them up and pin them to your body form, making sure they're not touching each other.