The first thing I do is try to get a handle on the libraries it pulls in (maybe spend a day just going through the high-level readme material for each one). Tha

The first thing I do is try to get a handle on the libraries it pulls in (maybe spend a day just going through the high-level readme material for each one). That will usually tell me where to start looking for the entry points where I might want to start modifying things. After that, I give myself a series of small functionality changes to implement, kind of like capturing a bunch of little flags. After doing that for a bit I usually have a decent idea of how things work, and it's easier to go forward, at which point I can dig into the relevant parts of the codebase with more confidence. The first few mods are inevitably disgusting hacks, so don't pick anything you want to keep for your first couple of goals. It is pretty easy to go back and do them right once you've got your head around the rest of the project if you do end up wanting to keep them though. I've used this method on some decently large C++ and javascript projects (around 100k-200k lines) and it works pretty well for me. I don't learn very well by just reading the code, but doing the little mods seems to make it stick.