Providing constructive feedback for peers is a powerful art. Doing it well means giving the gift of a new perspective to someone else, and the karmic rewards are sure to come back to you.
Be sure to articulate both the strengths and weaknesses of a draft. It can also be very helpful to mirror back to the author what you understand their primary point and purpose to be. Be sure to refer to the text of the assignment to ensure that your recommendations are aligned with the assigned expectations.
The following are some questions you might ask yourself while undertaking a peer review:
How would you summarize what the creator is trying to accomplish in the draft? Reflect that back to make sure your reading matches the creator's intent.
What do you think is working best about the draft? Choose some specific features that any revision should be sure to build on.
What kind of revision should the creator focus on? Are high-level, structural changes necessary, or should the creator focus more on smaller-scale adjustments? Pick one of these to focus on, and indicate to the creator why you're making that choice.
What audience do you think the creator aiming to reach, and to what effect? What choices help or hinder persuading that audience? How well does the draft accord with the rules or norms of the relevant genre?
Where would you recommend focusing revision efforts? What are the highest priority concerns, in your view?
What are some concerns that are smaller, but that should nevertheless be considered?