Once the shells are printed each component is placed inside and soldered together following the schematics.
The biggest part of soldering was connecting all the digital switches to the GPIO ports. Not a complex task but delicate and trivial, especially when the space for cable management was restricted.
The GPIO pins are connected to each and every switch in order to read the digital inputs.
Soldering everything with the 2 3D printed case halves side by side is the best option, as the folded cables add up fast and the shortest they are the better.
The screen required a small modification in order to be powered correctly and the battery was a little bit too big for the enclosure (an extra 2mm needed to be squeezed in) but everything worked out fine.
After a couple of hours, once assembly was complete ,we tested the whole assembly first by checking the power was charging the battery and then firing up Retropie from the SD card.
After a successfull login (A monitor and a keyboard or a SSH terminal is a must) we set up the GPIO ports using the Adafruit retrogame script (you can find it here).
After every button was working along with the battery and audio it was time to put everything together with screws.