The gamepad was designed in EagleCAD and available to download and modify. It's also a shared project on Oshpark.
Let's start assembly by putting the gamepad together. Gather up 10x 6mm tactile buttons and a 40pin IC box header.
Install IC Box Header
Insert the box header into the header labeled pin on the back of the gamepad PCB. Make sure the "notch" opening is pointing towards the "header" label. Secure it in place by adding mounting tack to the sides. Now we can secure the PCB to a Panavise to assist while soldering.
Solder IC Box Header
Flip the gamepad PCB over so the header pins are facing up. Now heat up the soldering iron and apply solder to each of the 40 pins on the header. Ensure the solder joints are clean with no cold solder joints or blobs. They should look like Hershey's Kisses.
To make it easier to insert the buttons to the PCB, use a pair of flat plier to straighten out the four leads of each button.
Install 6mm Tactile Buttons
Insert each button into the through-hole spots on the gamepad PCB. Once installed, place your hand over the buttons and bend each of the leads inward so they "bite" onto the PCB - This will help hold them in place.
If you are using the elastomer buttons, they're still possible to use, but they wont sit perfectly flat. That's OK just make them as flat as you can!
Solder Buttons to Gamepad PCB
Secure the gamepad PCB to a Panavise and solder up the buttons. Make sure solder joints are clean like you did on the pins of the IC box header.