This tutorial will show you how to turn Arduino into HID controller (in this case simple keyboard).
This tutorial is for Linux users. It will cover step by step how to flash an Arduino with miscellaneous firmwares (it’s for absolute beginners, you should know Linux quite good to perform this). I use cheap cloned board called Mega Keyes model Mega 2560 R3 with Atmega 2560 16AU. You can also use this method to unbrick your faulty Arduino.