Take notes (external to the code) on everything you learn about a given module, whether code/flow analysis, insights on input / output / objects, visible intera

Take notes (external to the code) on everything you learn about a given module, whether code/flow analysis, insights on input / output / objects, visible interactions, and what it reveals about how project-specific libraries are used. Otherwise, it's so easy to have an "oh, that's what that does" insight and lose it later because you assumed you'd remember it.