The process of writing reviews on Mesh doesn’t really change, irrespective of your role in the organization or the role of the person you’re reviewing.
The only difference will be in the review type that you'll see inside the review module.
As you can see here, for Troy, there's a Self Review form where he has to review himself and an Upward Review form where he has to give a review for his manager. In case of a peer, you'll see a Peer Review form here.
For a detailed walkthrough, please check the documentation for writing reviews.