Personally, I had some courses on computer architectures, and those covered the theory of how CPUs are constructed, how they run programs, etc. I decided to write an NES emulator. So: - Find Wikipedia articles, and learn that it used a variant of the MOS Technology 6502, which was used in a lot of computers in the 80s.

- Find some digitized assembly programming manuals from the time (I think the one I used was distributed with the Commodore 64, and ended up having several typos introduced by OCR).

- Write a tool to recognize, decode, and print out an operation when you feed it a little data

- You basically need to set up a loop of fetching instructions, interpreting them, then doing what they say. An actual CPU runs in a similar loop, and it generally doesn't stop until power is removed,

I think that after the classes I took, I read a lot of what other emulator writers said. This article is a basic look at the structure of an emulator, the theory behind them, and some different designs: http://fms.komkon.org/EMUL8/HOWTO.html