Find a proper enclosure. I have used a clear plastic box that originally housed baby's pacifier.
Drill two holes. One larger for the switch and one smaller for USB cable
Wire the switch. I used digital pin 9 and GND.
Load firmware into Arduino Micro Pro. Use the sketch code from Github project.
Test if everything works:
and secure the hardware inside enclosure.
Optionally: modify the code. I have already added code to handle a second switch, a toggle switch with 3 positions (up, middle, down) that you could use for additional two more keyboard actions.