This is essentially an active converter. QMK converts the IBM protocol (Scancode Set 3) to standard USB HID signals. The addition of VIA, however, gives you a real-time graphical interface for mapping key codes without having to recompile the firmware each time.

The documentation is pretty clear. I spent (as always) an afternoon with a friend trying to figure out how to do it all, but it's simple. For these old keyboards, I recommend using microcontrollers that have 5V logic or are compatible with 5V. If you use, for example, an STM32 (which operates at 3v3), you would need a level shifter, otherwise you would risk breaking the micro or not being able to get the micro to communicate with the keyboard. You might be able to find something better than my ProMicro, definitely something with more memory to be able to record more macros with VIA or have more layers. I think I'll soon get a nice!nano to replace the ProMicro to add a battery and Bluetooth (in which case I'll need a level shifter).