Drill a pilot hole and then a 3/8" hole in the bottom center of the body, for the USB port. Clean up any stray plastic with a rotary tool or sand paper.
Drill a second hole just below this one for the on/off switch. Don't worry too much about making it fit exactly - you want the hole a bit bigger than the switch and port so we can get them through later.
For the hole into the neck, I'm using a 3/8" spade bit with a long drill extension. Slip it through the bottom hole and carefully drill through into the neck.
This hole needs to be big enough to squeeze the two NeoPixel Skinny strips through. 3/8" seems to be just perfect.
Now it's time for the fiddly part of placing the light strips inside the ukulele. I made judicious use of needlenose pliers, zip ties, q-tips, and anything else I could find to wrangle the strips into place.
I had the most success using a bit of coathanger wire, straightened out and slipped through the bottom hole and then the neck hole, giving me a straight shot at pushing the wires up as far as they would go.
To hold the neck wires in place, I drilled a tiny hole on the back of the neck and emptied a small tube of krazy glue into it.
My neck wires don't lay quite flat, but I actually like the wavy effect I get from them quite a lot.
During this process, my neck strips somehow got reversed and placed on opposite sides from what I intended. If this happens to you, no worries - you can fix it in the code with pixel mapping. Check the Software page for instructions on how to do this.
Once the neck wires are in place, plug your battery and USB cable into your Feather stack. Use a rubber band or zip tie to stack the battery underneath the Feather. Make it look as tidy as possible, since it will show a bit through your ukulele.
Cut a piece of industrial velcro to the same size as the battery. Stick one side to the battery and use needlenose pliers to stick the other side to the inside back of the ukulele, as far down as you can reach through the sound hole.
Slip the feather in through the sound hole with the USB cable facing the bottom of the ukulele and press it down onto the velcro.
Now it's time to use your coathanger tool again. Bend it into a hook with your pliers and reach through the holes you drilled to pull the USB port and the switch to the outside. Take your time and be careful not to pull your wire connections too hard or they might break!
Next we'll mount these two components in place using a bit of thermoplastic. Use a heat gun to melt a bit of this down until it gets warm and squishy. It turns transparent when it's moldable, but be careful - it can get hot!
Squish some thermoplastic around the USB port and the switch and smooth it down so your components are more-or-less flush with the bottom of the ukulele. Be sure to push some of the plastic through the hole to the inside, so when it hardens it will grab in place. These components will need to stand up to pushing, so be sure they're pretty solidly mounted - and this thermoplastic seems to reject any and all types of glue, so a physical blob on both the inside and outside is your best bet.
Finally, glue the side-light NeoPixel strips along the inside edges of the ukulele body using silicone adhesive. Silicone adhesive is just about the only thing that will stick to the silicone coating on the LEDs.
If you don't have this or can't get a hold of this, you can take the silicone casing off and glue the strips down with E6000 or another glue, but if you're leaving the protective cases on, you'll need to use silicone glue.
I found my coathanger tool to be the most effective thing for spreading glue and pressing the NeoPixels into it. Wipe off any excess glue right away; this stuff dries rubbery and will be hard to get off later on.
Let everything dry, then shake your ukulele around a bunch to make sure nothing rattles before putting the strings back on.