You'll need:
For the Arduino, just open the Formula_GT.ino in your favorite IDE (ArduinoIDE will do just fine, eventually add the Joystick library and upload the code to your Pro Micro). You can tweek the name of the board (see how to boards.txt, the board.txt is my example).
Optional :
I made a custom PCB, so you could send the gerber files to your pcb provider (i used jlcpcb.com) and make one. Then you'll need to solder every pins to the PCB (check the photos) and the double-end dupont line to the buttons/switches
You can download the gerber file here : https://drive.google.com/file/d/19pEmVKapCboTJWVD-D5RFSGa5o8hH_UQ/view?usp=sharing
You'll need:
The author hasn't provided the model origin yet.