Unless you have the tooling and knowledge to manufacture precision parts like swiches and stabilizers, and integrated electronics like a microcontroller, it would be very hard to 100% DIY a keyboard.
The most DIY I've ever did was to design, 3D print, handwire, build and program a split ergo keyboard based around a Teensy 2.0 microcontroller and Kailh Box Jade switches.
An intermediate but still very interesting route would be picking matching parts from vendors like kbdfans or kprepublic, with your choice of enclosure, PCB, switches and stabs, lots of soldering and testing, and some QMK fun to round it off.