learn assembly, learn how it maps to opcodes. learn microelectronics and learn about cpu, registers, memory layout, cpu flags, etc. it becomes obvious.

i wrote a 6800 emulator just from having this background knowledge and no reference in my college years. I did it because I didn't want to go to the labs early saturday morning to fight over the few 6800 hardware. I could write my code, run, debug, show up just as the lab is ending and hand in my work and go home.