Find the female side of your waterproof connector. Slide the included cover nut onto the cable with the open side toward the connector, as shown.

Slide the white nut from your cable gland onto the connector facing the other direction, with the open side toward the bare wires.

Open up your GoPro case. Use a 15/32 (12mm) drill bit to drill a hole in the side of the case next to the on/off button. Get it as centered as possible so the nut has room to screw down flush with the case. I found it helped to drill a smaller pilot hole first to keep my drill from wandering.

Screw your cable gland's main section firmly into the hole as shown, with the shorter side on the inside of the case. Thread the included plastic hex nut onto the outside and tighten it with a wrench.

Slip the  bare wire end of your waterproof connector into the cable gland from the outside, making sure not to dislodge the foam insert. Adjust it until just the bare wires are sticking out, with the main part of the cable inside the gland. 

Double check that you've got both connector nuts on your connector: the white one from the cable gland, ready to screw to the case, and the black one that came with the connector, ready to screw to the male end of the connector.

Now that we've got a waterproof portal through our case, we can connect the 3-pin NeoPixel connector to the wires on the waterproof connector. Trim the yellow wire since we won't be using it. Connect white to white, red to red, and black to black. Cover the connections with heat shrink and solder.

Place the battery on the bottom of the case with the foam covering facing upward. Plug the NeoPixel connector into the Prop-Maker wing and nestle the Feather and Prop-Maker on top of the battery but leaving space around the two buttons on the case.

We'll use Thermoplastic to position our two switches inside the GoPro case to take advantage of the case's onboard buttons, so we can change modes and turn the board on and off without opening the case.

Use a heat gun or a pan of hot water to melt a small handful of plastic beads. They'll turn clear when they're ready to mold. Be careful - they can get pretty hot! Don't burn your fingers any more than necessary.

Position your momentary switch behind the button on the top side of the case. Mold some thermoplastic around it to hold it in exactly the right spot so the switch gets activated when you press the case button.

It's helpful to have your LED strip connected up so you can be sure the button is working. Let the thermoplastic cool to hardness once you've got it lined up.