One leg of the tactile switch will wire to the '-' pin on its corresponding neopixel. The second leg of each button will get wired to a data pin on your microcontroller.
The NeoPixels will be wired in series, connected to + and - and another data pin on your microcontroller.
Clip the button's wires to your Gemma running the buttoncycle test code. Your button should turn on with the first press and change colors with each subsequent press.
If everything is working, add a dab of glue inside the cup to hold the button and neopixel assembly down tight.
Repeat until all your buttons are wired, glued, and tested.