Scikit-learn is the machine learning library for Python. Thomas Fan, a core developer for scikit-learn, gives a presentation on how to review pull requests. Specifically, the 3 components are:

1. The mechanics of code review on GitHub.
2. The social aspects of code review and how to effectively give feedback.
3. The technical aspects of reviewing a pull request.

It's an informative video, with a great Q&A session at the end.
Thomas Fan: Reviewing Pull Requests