Run it under debugger and go thru whole codebase a few times and you'll start getting proficient at it
I've recently did it on huge, very very specific codebase and after 2-3 months I understood it (what to add where, not just what's happening where) relatively OK
I've recently did it on huge, very very specific codebase and after 2-3 months I understood it (what to add where, not just what's happening where) relatively OK