Comment by t-3

Most of the work involved in emitting code deals with somewhat arbitrary hardware details. Pretty much the only way is to get familiar with your target hardware features, instructions, and platform ABIs. ISA manuals all have sections describing these, they're pretty much always very dry and "linguistically rigorous" so it's not easy to read but everything you need is there. Actually emitting instructions is pretty boring and mechanical outside of some considerations and subtleties with hardware-specific optimization, OS calling conventions, register selection, etc.