Use the gerber files to order the PCB from the company of your choice. You'll also find the schematics that will allow you to modify some aspects and route your own PCB if wishes.

You'll find the BOM that you can use to make the order. Some of the components are pretty small and will require some skills.

Connect the board to a PC using a STlink in order to flash the MCU with the bootloader file.

Connect the board to your PC using the mini-USB port. A new drive will appear on which you'll be able to copy the uf2 file of the game. The board will reboot after that.

Import the .stl files in your slicer to prepare the print. Once all pieces are done, you'll have to do some adjustments in order to have all the pieces fit together. You can add some hotglue under the buttons to make good contact with the switches.

Enjoy yourself in customizing the case. A good process to get rid of print imperfection is to sand with differents (eg from 600 to 1000) under water, add coats of primer, add coats of paint and add varnish.