First, RTFM. If there's documentation, I read it. README, CONTRIBUTING, whatever is available really. Then, I start hunting through the codebase. Sometimes des

First, RTFM. If there's documentation, I read it. README, CONTRIBUTING, whatever is available really. Then, I start hunting through the codebase. Sometimes despite people's best efforts at compartmentalization, there's one or two files that are the heart of the project. Depending on the project they'll be different things. For instance, TypeScript has checker.ts, which contains the core typechecking logic. Ruby has vm.c, compile.c and parse.y. If that's the case, that's actually very helpful, as I can spend the majority of my time in one file. To aid in this hunting, I use a few tools. Stuff like grep and find (although I prefer ripgrep and fd) are a huge help, cause you can search through large codebases with relative ease. IDEs are great too. I particularly like being able to goto definition, then go back, then go forwards, etc. Switching between call site and definition makes understanding functions easier. I take notes on occasion, although I don't always reference them. It's more to process what I'm reading. I try to write notes about types, functions and files. I do it in org mode and embed urls so that I can link definitions together. Definitely run the code as soon as possible. Then add print statements and see where they go. I've used flamegraphs on occasion to see the stack trace.