Critiquing another person’s work isn’t as easy as it sounds. To write a critique that’s not only thorough but useful, you need to include (and avoid) quite a few things.
Tell them what doesn’t work. This might be the most obvious part of critique, something that everyone knows to do. However, it’s not enough to say, “I don’t like this” or “This is bad.” The thing about critiques is that they’re constructive, which means providing the author with a reason for why you don’t like something. If they don’t know why you think something sounds weird or doesn’t make sense, then they’re not going to know what to fix, and they won’t be able to learn. It’ll come off as simple hate, since it doesn’t offer anything for the author to work with. So always be sure to give specific reasoning.
Tell them what does. A lot of people seem to believe that writing a critique means only pointing out what’s wrong, but they ignore what’s done right. It’s just as important for the author to know what they do well as what they don’t. When you put emphasis on the good, you’re not only showing the author encouragement, but you’re also allowing them a chance to focus on and strengthen what they’re already good at. Again, avoid being vague here. It’s specifics that are going to be the most useful.
Don’t rewrite the story for them. This is not your story; it’s theirs. Unless they specifically ask for ideas or help with a section or plot idea, do not tell them what they “should” do. That’s for them to decide and discover themselves, and it can seem invasive when you try to make huge changes to their work. Showing them how to rewrite a sentence to make it flow better is fine, but telling them how to rewrite their villain is not.
Don’t focus on line-editing. When you’re critiquing something, chances are this isn’t a final draft. Therefore, line-edits don’t really matter and just take up both of your time. While they’ll certainly require line-editing at some point, it’s most likely not now. It might be worth mentioning a recurring grammatical error, but otherwise, you should probably lay off.