How to start reading the source code of a open source library or project

I am very interested in contributing to the open source community and hope to be eventually able to write and share my own projects with the community too. But it feels very overwhelming if I try to start reading, analyzing and learning the code of a well established project in the beginning, although the project may be a small one (a JS library under 7k LOC for example). I don't know if I should read the code from entrance point to finish (which seems to require lots of patience), or should start from the usage POV (how those features are implemented).

Does anyone have some advice for reading and analyzing a project?

Thank you!