My favorite technique is to run the code, place breakpoints in interesting locations and manually type out psuedocode descriptions of each function up the calls

My favorite technique is to run the code, place breakpoints in interesting locations and manually type out psuedocode descriptions of each function up the callstacks. That's the fastest way I know to get an understanding of the structure of a new code base. Barring running the code... I prefer to start with a quick scan through the foundation code. What does the math library look like? How about the file and network I/O? Next, locate the top-level main loop, then the top-level Update Everything/Render Everything functions. At that point, you'll understand the general theme of the code and you'll have spotted lots of interesting avenues to explore.