The ATMEGA 32u4 chip has build in USB 2.0 module allowing this microcontroller to become a dedicated USB HID. Some simple code can turn this into a mouse, keyboard and even a joystick.
The video example demonstrates an arcade/keyboard controller (jump to video section) built with a 32u4 based microcontroller. A simple screw terminal breakout board is use to terminate wires. In some previous videos I used a Kade device to build+program a keyboard encoder; however, these Kade devices are no longer available. It is worth mentioning they are built using the same chips- the 32u4, so why not just write our own code on a cheap development board.(In the future I will be doing a dedicated video to explore the possiblities of the keyboard encoder with an atmega32u4.)
The code used in the video is below. Check the sparkfun tutorial for even more help/reference questions.