The comment should be at least 1 page (double-space) long, or something equivalent. You can write them directly in the body of the email if you want. Try to say something substantive that will help the author of the paper. I expect you to provide constructive comments. Try to think about what comments you would find constructive if someone else wrote it for you, and try to do just that! Maybe you can recommend discussing a new reference, clarifying the thesis, clarifying the argument structure, explain a case study from some particular science more transparently, etc.