Don’t exaggerate -- show where and how they can improve. Don’t put the candidate on a pedestal. Not only does it not look plausible, it will also set expectations for them that will be next to impossible to meet. If they have an Achille’s heel, don’t exaggerate it, but don’t hide it, either.
Writing a letter of recommendation can be intimidating, especially because the person you’re writing it for is counting on you to pull through for them. Fortunately, there’s a general formula you can use to write a letter of recommendation that is both professional and effective.