Custom split ortholinear-ish keyboard

How to use ATmega16U2 on an Arduino as HID keyboard

To enable keyboard functionality

To edit sketch

Alternative firmware updating method

In case of "AtJnilsp Message - AtLibUsbDfu.dll not found"

This means we need to manually install a driver. This video shows you how to do this.
In case the video is unavailable, here's a summary:

Other references: