A guide for reviewing code and having your code reviewed. Watch a presentation that covers this material from Derek Prior at RailsConf 2015.
Understand why the change is necessary (fixes a bug, improves the user experience, refactors the existing code). Then:
Reviewers should comment on missed style guidelines. Example comment:
An example response to style comments:
If you disagree with a guideline, open an issue on the guides repo rather than debating it within the code review. In the meantime, apply the guideline.