I am interested in building Android for Source for a Galaxy S3 (but just knowing how to do it in general will be quite helpful)
I know how to do it for all the nexus devices in general, but I was wondering how to do it for some of the other phones. I know that it is possible because CyanogenMod and other modders do it. Usually when they announce them they include the source for the kernel. How do you put the pieces together? Getting the android source code and the custom kernel together in something that can be flashed.
Generally it is not trivial. You need some binary files for hardware support (camera, WiFi, NFC, etc) and you need to figure out which those are and how to pull them from the device. The kernel is GPL licensed, so the the manufacturer will release their version/patches at some point, but you might have to wait. Then you have to create device build files to put all those together in an image for the particular device. Your best bet is to get CyanogenMod source and see how it is done in practice.