31
First things first: I'd check to see if there's an Unofficial image. Consider that and, while you're at it, also consider the risks or making an unofficial build your daily driver. Phones are now vital infrastructure, and you can really strand yourself if an unreliable image bricks your device, or throws some weird comms error that strands you in an airport at the wrong end of your journey.
This is Advanced Technique, no matter how you dice it. One of those One Does Not Simply... ๐" things. Caveat emptor, here be dragons, ...
*** YOUR WARRANTY IS NOW VOID. ***
Still reading? Brave soul.
Broadly, you'll need the factory image for the device to extract the binary linked objects ("blobs") that drive the actual hardware: the modems, camera, SIM card and whatnot. The necessary blobs would be listed in the developer guide.
Then you'll need to configure and build the system image. You can do this on Windows, but it's certainly easier on some sort of Linux platform.. If you don't already have one handy, you could run one in a VM or Vagrant image (if windows) or whatever Apple the equivalent is for that platform. You'll need to build the compiler environment to wth mukti-arch support for cross-compiling your phone's architecture. Oh, you'll also need to know tour phone's architecture, I.e., what SoC and firmware it uses.
Finally, youlo need to unlock your bootloader and either find or build a compatible recovery image to load the system image, and the Gapps if you're using them.
Pretty much ALL if this is an exercise for the reader. The reason there aren't more official I mages for less popular platforms is because maintaining an official image is A Lot Of Work. For many maintainers it's a labor of love.