For handwiring, Joe Scotto’s videos on Youtube are excellent. For something a little more old school, but maybe more accessible theoretically, Matt3o’s walkthrough from the before-COVID times is still viable.

If you want to design a PCB, KiCAD is quite usable now, and there are plugins for keyboard switch footprints and to take Keyboard Layout Editor data and place them perfectly. For me, with no EE background whatsoever, the biggest mental hurdle was understanding the design paradigm “electrical schematic first, physical layout next”. I’ve been content to leave my PCB level at the point of making the entire keyboard PCB dumb and leaving the brains of the thing to a cheap and replaceable Raspberry Pi Pico (I like the clones that use USB-C). Physically, they’re basically larger versions of this tutorial from adafruit, though I use more standard keyboard firmwares. The other benefit of a PCB is that the minimum order most places is five, so with a couple of design choices (e.g. include mounting holes and rounded corners) and some PCB standoffs, you can have two usable builds with no case at all, and a spare PCB.