I tend to use a hybrid approach, but In general I try to identify the entry point of the code which will lead me to the core datastructures and possibly event loops that act as a central hub for any other code that is called.. That is I look for some kind of dispatch pattern that Integrates the rest of the system, routing and calling different code when needed. Once you identify this "hub" you will have a good mental model and the system and its high level components. From there you can delve into different subsystems and slowly tweak and make changes to be sure a code path does what you conjecture it may do. Using a debugger is helpful at certain points to explore depth of the code.. When you can get a small tweak working as expected you probably have a decent starting model of the code base that you can easily add to.