Generally, for an unfamiliar problem domain, you just try to do it how you can and if it feels hard - try to come up with another way to do it until it doesn't.

Generally, for an unfamiliar problem domain, you just try to do it how you can and if it feels hard - try to come up with another way to do it until it doesn't. For an intermediate level programmer there is unlikely to exist a problem domain where you are proficient enough to just read the sources and understand how everything works right away. Everything is pretty much new and unfamiliar. So, you'll need a good high level view on how everything works, step by step. Go with a call tracer. It would take some time to get it to work, but even with that there is nothing, that could beat it in efficiency of digging into a new codebase.