I never know where to start looking, specifically if its a language i am not too familiar with i have no idea where to start and sometimes i have no idea where the program execution starts.

Is there maybe a ladder of small to large open source projects that can get you there?

For example i have no idea how to begin reading the Flask(1) open source code.

What approach can i take to get to a point where i can analyze a project like Flask and get something meaningful from it?

(1) https://github.com/pallets/flask