Are you looking to build a keyboard from a kit, or design one from scratch?
SplitKB.com has forked a few popular open source keyboard designs like the Corne or the Sofle. Even if you don't end up purchasing their kits, they have some excellent build guides and documentation. If you're a keyboard or DIY electronics beginner, it's a good way of seeing what sort of techniques and tools you'd be in for when soldering things like microcontrollers, switches, and diodes. https://docs.splitkb.com/product-guides/aurora-series/build-guide
If you're interested in designing a keyboard, I'd recommend looking into the keyboard layout editor Egogen. https://github.com/ergogen/ergogen