[bubblehack3r]

Run the code. This sounds so trivial but it's not always that easy. Once it get's running, I discuss with users of the code (be it customers or developers) and try to understand what they use it for and look for that same functionality in the code. Usually by this point I know where everything is and how to make changes if needed.

[windowsworkstoo]

Yep, underrated advice. Run it, read the code, fiddle some bits and make sure what you fiddled matches your mental model.

Honestly, learning to read code and execute it in your head is a super power

[miohtama]

Even better than running code is running tests.