Trace one real request through a codebase before you change anything

Pick the simplest user action (log in, load a list, submit a form) and follow it from the route to the database and back. Write each function name in a scratch file. After one trace you know where the seams are, and the rest of the repo stops looking like a wall. It took me a week to learn this, which is embarrassing, but it's what I'd tell myself on day one.