Authors deserve to have a professional-level critique of their work, noting its strengths and weaknesses. Don't forget to think about the strengths of a manuscript. Mention the strengths in your review, even if you aren't recommending that the manuscript be accepted. This can make your review more credible to the author, who in turn is then more likely to accept your suggestions for improvements. Think seriously about what advice you might give to the author in an effort to help out.
Many authors are happy to take suggestions that improve their manuscripts.
Be specific about needed revisions. This helps both authors and editors. Most journals ask you to review a manuscript within a month. Observe that deadline. One of the most irritating things in the operation of journals today is the long time between submission and reaction.