It's not that complicated, it's just a slightly more accurate mitre box with a sliding bed to advance the fretboard between fret slots, the board is CA/masking tape secured to the bed centreline. The toggle clamps lock the whole assembly down while you cut.
It's a bit of a work in progress, but the first test cut seemed good. I'm going to prepare a pine test blank to do the fret measurement test and the fine tune on this cracked rosewood board, with some inlay work, prior to getting to grips with the final ebony board.