How extreme do you want to go?
How much money are they willing to spend?
There's a few ways to go here from most expensive and most amount of work to cheapest and fastest.
You can cad the whole case and pcb and send it out, this can literally take thousands of dollars the better part of a year before you have a working keyboard.
You can start with the KLE (Keyboard Layout Editor), import that into some other tools and send that off to a shop to be laser cut. As before you can hand wire or make a PCB. This can take weeks or months and a couple hundred dollars.
You can also 3d print it.
As before you can hand wire or make a pcb, but 3d printing offers another option, you can 3d print a "pcb" and embed hot swap sockets into a printed plate. You can start with the KLE and other tools then convert to 3d print objects, but there's actually an easier way in 3d printing, you can take existing designs found on 3d print repositories (thangs, cults, thingiverse) and import them into CAD or (even easier) Tinkercad. Tinkercad you can learn in minutes and basically works almost like Lego. Export it, print it, then hand wire. I have a bunch of tips in my signature under "PF68" which also has links to even more info on Thingiverse. Don't use the PF68 parts, it's functional but over complicated and for someone new to CAD and keyboards but does have a lot of info regarding plastics, infill and sound. With this method you can have a working keyboard in days or a few weekends, and only costs a spool of plastic and various keyboard parts. I think I spent $110 for everything on my 65%, that's for plastic, caps, switches, swaps, controller... You can't really get much cheaper. You could even buy a printer and still cost less than getting laser cut plus an actual pcb.
As for electronics, you'll not be able to use a Pro Micro (the cheap option), not enough I/O, which means a Proton C is probably your best bet.
Hand wiring is tedious but not difficult.
