Quick post today. (Since I'll be gone on Monday.) I thought a post on critiquing might be helpful.
Real fast, there are two basic types of critiques, 'line-by-line', and 'general thoughts'. I prefer to call the latter 'thought dump' but it's not as official-sounding... Line-by-line is as the name implies, where one carefully reads through the excerpt of writing and comments on things throughout, down to the nit-picky details. (Well, depending on what draft it is.) General thoughts is a bit more, well, general. For that one, a person would ignore the tiny issues and make blanket statements about what the excerpt needed or lacked. It mainly gets the big problems. Line-by-line critiques are usually better for work that's further along, and general thoughts can be helpful for a first draft.
Now that that's sorted, let's get into actual tips.
1- No grammar. If they wanted a grammar critique they could have asked their mother. It truly isn't helpful; grammar isn't the meat of the story at all. I’m guilty of pointing out spelling and things... I do it because messed up grammar makes things impossible to read for me. Don’t be like me. In a line-by-line, you can sometimes get away with fixing grammar here and there since that type of critique is often fairly dense in other comments, but as a rule, unless someone specifically asks you to point out grammar problems, DO NOT DO IT.
On the other hand, if you submit something to be critiqued, try and catch as many of the grammar issues as possible before sending it, believe me, it helps the critiquer a lot. (Yeah those squiggly lines under stuff in the word doc? They usually mean something's off.)
2- Choose a focus. Ask the person you're doing a critique for if they have a certain element that they most want to improve, and examine that the most. Characters are my specialty, and I enjoy character-driven books as opposed to plot-driven, so I find myself making comments that reflect that, where I point out ways to help people get more into characters' heads and things of that nature. It's impossible to critique everything about a novel, so find your niche and help in your way.
3- Analyze the genre/target age/setting and critique accordingly. Make sure the things you point out aren't specific to your preferences. There's nothing wrong with saying "this isn't my type of book, but if you added sports/romance/clowns/talking dragons/oompa loompas/whatever it would be more appealing to me." However, it's important not to force someone's writing to be something that it isn't. If you like sci-fi and are critiquing a medieval fantasy, there's a slim chance time travelers are gonna zoom in on a space ship. (Although by all means, give people crazy ideas like that. It might be helpful.) Likewise, a middle-grade book is going to be geared towards that age, and while it can still be enjoyable for older people, it might not resonate with them as much as with the intended age range.
4- Use your discretion with sarcasm. I am a sarcastic little human. However, the first time I got a sarcastic critique, I totally felt mocked and was a bit offended for kind of a long time. (I now find sarcasm in critiques entertaining, so all you snark sharks have full permission to go all out on my work.) For me, deciding whether to tone down the sarcasm or not depends on the person. If I don’t know them very well, I’ll definitely toss sarcasm out the window. If I sort of know the person, depending on their personality I might put in a bit. And then with my very good friends it’s just like a wall of sarcasm. But when in doubt, throw it out.
5- Be truthful but tactful. This goes along with the previous point. Be considerate with how you present comments. Don’t dismiss things as “well this is the most horrible sentence I’ve ever read.” Remember that writers are people too, and for some people it takes quite a lot of guts to show writing to someone else, and even more to accept others' feedback on it. Writing is a little part of someone's mind, soul, heart, so be very careful with how you suggest changes. That being said, don't sugar coat stuff. Don't say something is good if it needs alteration. Be gentle and thoughtful with critiques. Of course, like with the sarcasm thing, once you know someone well enough you can get a feel for what degree of blatancy they can handle.
Overall, when critiquing, keep in mind the Golden Rule and remember you're trying to help the other person's writing improve, you aren't there to gratify yourself by pointing out other peoples' flaws.