I'd say just being curious, reading and tinkering a lot. I recently wrote a GameBoy emulator to give myself a better understanding of the inner workings of CPUs. It's anything but precise, but it passes all the CPU instruction test ROMs and it's able to run all my childhood games so that's mission accomplished for me. Maybe I'll try to emulate something more complex next.

Writing an emulator is very frustrating at first (when nothing works) but it eventually gets more rewarding as you get to the point of meaningfully running preexisting software. IMO every programmer needs to write one at some point.