I tend to try to get the big picture. What is the overall structure of the program? If it's a game, for example, then what layers are there between the user an

I tend to try to get the big picture. What is the overall structure of the program? If it's a game, for example, then what layers are there between the user and the display driver? Where does it set the screen mode? What is the progression of the drawing functions, etc. Sometimes you need only to figure out one thing. I.e. how do Bitcoin encrypt it's keys? Trace the user input into the bowels of the client and out into OpenSSL. In the process you would get an idea of how the onion is layered. If it's something truly massive like ReactOS or Linux kernel, then... No idea. In my case i just dived in and kept crashing it until i stopped crashing it. The best way to do it is to get ahold of a developer who knows the code, of course.