To start this project, I suggest securing the Feather to the cover of the 3D printed case. Grab the Feather board and place it over the standoffs. Orient the PCB so the micro USB port is facing the side with the "cutout". Line up the mounting holes with the holes in the standoffs. Insert two M2.5 x 5mm flat Phillips machine screws into the mounting holes and fasten them tightly with a Phillips screw driver. Fasten until the screws heads are flush with the PCB.
Next, we'll work on the installing the arcade buttons into the 3D printed case. Remove the mounting rings from the buttons by unscrewing them. Then, insert the body of the buttons into the cutouts on the case. If the tolerances are loose, you can apply pressure and press them into the case until they're flush with the surface. You can also screw them into the cutouts. Secure the buttons to the case by reinstalling the mounting rings.
Wires for Ground
Next, we'll need to cut some pieces of wire for making the ground connections. I suggest using 30AWG silicone cover stranded-core wire. Depending on the amount of buttons you'd like to use, you'll need to make pieces of wire accordingly. For my 2x2 button layout, I needed a total of 8 wires (just for the ground connections).
The length of these wires will vary, but I suggest making them at least 8cm in length. Longer wires are good because you can always shorten them. The majority of the connections will be for common ground. In my 2x2 button case, the wires varied in length, as short as 5cm and as long as 10cm. Use wire cutters to cut the wires to your desired lengths.
Next, we'll need to strip the wires to expose a bit of the strands. Use wire strippers to remove a small amount of insulation. The exposed strands of 30AWG wire are really thin and can easily fray, so be careful handing them. We'll need to do this to both ends for all of the wires.
Now we can tin the exposed strands of wire. You can this by adding a small bit of solder to wires using the tip of a soldering iron. This will make it easier to attach the wires to the electrodes on the bottom of the arcade buttons. A helping third hand can hold the wires steady and in place while you solder. I suggest grouping multiple wires to one of the grabbers so you can quickly tin them.
With our bits of wires cut, stripped and tinned, we can now work on connecting them to the buttons. I suggest securing the enclosure with the installed buttons to a panavise or similar. It'll make soldering much easier if the enclosure is mounted in place and stationary.
Tin all of the electrodes on the arcade buttons by applying a small bit of solder. Each button has a set of two electrodes, four in total. The first set is for connecting the LEDs while the second is for the momentary switch.
Arcade Button Connections
Here's a close look at the electrodes. Take note of the orientation and markings on the button to help you understand the connections. We'll be connecting all of the ground together so they're wired in series. This means we'll use wires to connect from one ground to the next. This prevents from having to wire every single ground connection to a pin or GPIO. It's much easier if we have a SINGLE ground connection going to the Adafruit Feather instead of each button individually. So the goal is to connect one ground wire coming from one arcade button to a ground pin on the Adafruit Feather. The rest of ground connections from the arcade button will connect to each other, essentially "sharing" the ground connection.
Test Button LEDs
If you haven't already, it's a good idea to test the LEDs in each button. To do this, you can used alligator clips to easily connect the electrodes to a breadboard. You could also use jumper cables. You can also use a coin cell battery to apply power to the connections. Once you've tested the LEDs and determined the voltage and ground electrodes, take note!