Help with S-Function builder

I searched the web and even found class material to use the S-function and the code generation. However nobody really explains it well. Steps are missing, lack of documentation and no good explanation in order to help doing custom codes. Please somebody could indicate a good material or a good source, even a book if possible that explains step by step what everything means on the UI of the S-function builder and how I can add custom libraries to work with microcontrollers like arduino, or straight with ARM, PIC, and so on....