Start by making a template of your hand. Place one hand on a piece of paper and trace all the way around it. Next, mark the placement of a NeoPixel on the tip of each finger, and the location of the Gemma M0 on your wrist. Draw the path of the circuit, as shown, starting from the Gemma M0, and ending at the last NeoPixel on the pinky.
Now, measure your line to determine the lengths your wires should be. Add 1 - 2" to each length for ease of movement. Add 3-4" to the wire going from the Gemma to the thumb, trust me, you'd rather have too much here than too little. For reference, my measurements were:
Gemma to Thumb: 8 1/2"
Thumb to Index: 8"
Index to Middle: 7"
Middle to Ring: 7"
Ring to Pinky: 6 1/2"
Cut power, ground, and data wires for each length needed. Label the wires with removable tape to keep everything organized while working.
Strip both ends of each wire. The pads on the NeoPixel Nano LEDs are super tiny, so you only need to strip a tiny bit of insulation off the ends of your wires.
Cut a piece of foam core or cardboard about the size of a business card. Apply double stick tape to one side as shown. We'll use this sticky surface to hold the NeoPixels and wires in place while soldering.
Slide the card into a Panavise Jr. (or just affix it to your work surface) and position it under your magnifier.
Soldering these tiny NeoPixels is delicate work. Don't linger with your soldering iron, it's easy to melt the plastic and destroy the solder pads. Be deliberate, and expect to sacrifice a couple of NeoPixels while you hone your technique.
Remove one NeoPixel Nano from its packaging and pick it up with tweezers. On the front of the NeoPixel, one corner is cut. Place the NeoPixel face down on your sticky card as shown, with the cut corner at the top left.
Carefully tin the pads with solder. A very light tap of the hot soldering iron on the pad should be enough for the solder to adhere. If you have a fine tip for your soldering iron, it can be helpful.
Tin the wires. Lay the tinned wire on top of the tinned pad. Use your heated soldering iron to briefly press the tinned wire to the solder pad, the solder will flow quickly. Don't linger! Repeat to attach wires to all the pads as shown.
The first NeoPixel (for your pinky finger) will only have three wires coming from it, but the rest will have six: data in, data out, two power, and two ground. Data in and data out are pretty straight forward, but here's my technique for soldering two wires to these tiny pads:
Use tweezers to twist the two power (or ground) wires together tightly. Tin the twisted wires. Lay the wires over the pad as you've been doing, and solder as before. Soldering the thicker bundle of wires will take a millisecond longer than the single wires, but you still want to move quickly.
Continue soldering the LEDs together keeping the wires in order, until you've connected all five NeoPixel LEDs.
Once you have all the NeoPixels soldered into a chain, you're ready to connect them to the Gemma M0.
Set the Gemma M0 in the helping hands. Solder your wires as shown: power to Vout, ground to Gnd, and data to D1.