Google is sharing “How to do a code review” as part of its engineering practices. Unlike many similar guides online, I find this document to be a lot more comprehensive. It covers both the technical bits of the process, as well as suggestions that improve overall team communications and efficiency.
A particular type of complexity is over-engineering, where developers have made the code more generic than it needs to be, or added functionality that isn’t presently needed by the system. Reviewers should be especially vigilant about over-engineering. Encourage developers to solve the problem they know needs to be solved now, not the problem that the developer speculates mightneed to be solved in the future. The future problem should be solved once it arrives and you can see its actual shape and requirements in the physical universe.