I start by downloading the processor documentation from the manufacturer and reading through the entire instruction set. That teaches me about the processor architecture in general. The next step is seeing how Windows uses it.

That part usually starts with digging out the Windows NT installation CD for the relevant architecture and extracting the NOTEPAD.EXE program. I choose Notepad because it’s relatively small, or at least it was relatively small at the time. Furthermore, I have an old copy of the source code, which makes the reverse-compiling easier. The source code I have doesn’t always perfectly match the build of Windows that the CD was created from, but it’s usually close enough.