Well, I'm not very good at this either but here's what I do. I usually work on modular projects where there are hundreds of files in the project. I usually skip directly to locating the file where I've to make amends (using a lot of grep. grep for function and object definitions, grep for usage patterns, grep for checking how to implement something). Thus, I learn about the codebase as I go along.

Sure, this is not the best practice, and unsuitable for many, but it's what works for me.