Entirely hardware. You just need to run the signals from the keyboard to the pcb of your system of choice. I don't know of any guides, because I don't know of anyone who's actually done it. Knowing how all this works though, Its not hard to understand the process.
a mechanical keyboard would be the easiest (it has easy solder points on the board) you would just run signal and ground to all of the keys you wished to use.
By no building I mean you don't need to build a case or buy special buttons or anything. You could take an off the shelf keyboard and something like a Cerberus or Cthulhu PCB and find a way to house it in the keyboard.