[Jach]

Besides the good methods others have posted, and a really nice method (if you have it) of having someone else familiar with the code give you a tour, you can also do pretty well just by brute forcing it.

Get a list of all the files, sorted however (`find -name *.foo` works) and start going through them top to bottom, or bottom to top if that's a more clear convention of the language. Maybe shuffle order a bit if you discover unit tests (nearby or asking a tool to cross-reference a call) to read the code and the test around the same time, but resist the urge to jump around too much or too deeply. Jot down short notes about what seems to be the main purpose(s) of the file, and move on. Keep going, keep track of what you've seen, your first goal is to do a complete survey of all the files and not get too distracted by fully understanding new syntax (Java annotations and Python decorators can both be understood as high level declarative tags even though under the hood they're quite different) or endless note revisions from new insights as you progress and start seeing connections or just finally understanding terminology ("wtf is a 'hero'?").

You'd be surprised how fast you can do a single (high level, shallow, skimming in places) pass even for larger code bases, by the end of it you'll also have found the/an entry point, and are in a better place for followup study or producing materials that can help the next person (like an architecture diagram that lists the files involved in each element, at least at that moment, or just some important cross references you've noted that a tool isn't necessarily going to make clear). And for easy code, a single pass may be all you ever need, even if you read it in a strange order. A completed puzzle is perfectly clear regardless of the order you put the pieces down.