There are a few things to keep in mind when giving your own criticisms to a story. Here are some basic guidelines:
1) Critique the story, not the person. Don't assume that a character in a story is the author's mouthpiece. Very often, as authors, we write from the point of view of people who, quite frankly, we find loathsome, particularly when we're dramatizing problems in society that we dislike. For example, I could very well imagine myself writing from the point of view of a child molester. Does that mean that I'm attracted to children myself? No, I would be tempted to write such a story probably because I want to dramatize just how vile I think it is. Yet many a new critic will assume so.
2) Don't prescribe. Point out problems that you see in a manuscript, not the solutions. The only caveat to this is: If an author asks for possible solutions, then point them out, but don't campaign for your solution. Remember, Dave's rule: "There are a million wrong ways to write any story, but there are ten thousand right ways." In the end, the author needs to craft the story that best suits his or her own individual taste.
3) Don't get emotionally involved with your critique. I've seen critics who will plead with authors to do something long after the critiquing session is over. I've seen authors who will politicize the issue, campaigning about it with other writers in the group, asking people to back the critic's opinion, and so on. A critic who does this needs to get a life. He or she needs to go write their own stories, not someone else's. If you've got one of these in your writing group—and you probably do—put the person on notice. Tell them to shut up or get out, then enforce the rule.
4) Don't nitpick. Pointing out errors in punctuation does not a critique make. Sure, point them out on a manuscript, but look for something more constructive to say. Look at larger issues of characterization, plotting, setting, dialog, audience analysis, suitability toward the target market, failures of imagination, and so on. Those are the things that need to be discussed.
5) If you don't have anything to say, shut up. Sometimes a story works just fine for its given market. If you feel that way about a story, tell the author, "This is great for the X market." Then shut up. Be aware that this in itself might offend the author. I can't tell you the number of times when I've read great stories for the young adult market where the author thought it was for adults. I've also seen stories that looked like porn to me, and the author thought it was for a general market.
6) Set time limits on a critique. Give three or four minutes, tops. This forces the critic to nail the top two or three items that he sees wrong with a story. If an author gets half a dozen people commenting on a story, and each person nails a couple of items, that's usually enough of a critique for a session.
7) Set a limit on the number of critiques a story gets. Some authors will try to workshop a story to death, going over it year after year. You know what, folks? Forget it. Any story that you've seen more than twice has a problem with "failure of imagination." In other words, the writing may be fine, but the basic elements that make up the story are probably just not powerful enough to justify publication. For example, I've seen many an alien romance story that worked just fine—but they were too much like all of the other alien romances that I've seen. The author needed to bump something up—the world building, the alienness of the alien—almost anything, in order to get a sale.
8) When you critique a story, write on it in ink—then sign your name. This is important. As an author, I want your first impressions. For example, when you read about a character and you don't like him in paragraph one, but you grow to understand him and like him by paragraph three, I want to know. Very often I work toward creating such effects. So it's helpful to get your first opinion on a story. In fact, it's far preferable to a critique that analyzes the story days or weeks after the story has begun to sink in. I say this because I often write stories that I know are unsettling and yet which readers will come back twenty years later and tell me that it was a landmark story for them in many ways, one that resonated for years afterward. The problem is, a story won't resonate twenty years later if the reader doesn't make it through the first reading. So I want to know if there are things in the story that act as barriers to your enjoyment that are unintentional. Even if I have things that are unsettling, I might want to know if they are "too unsettling." For example, is my villain so creepy that you were forced to quit reading on page twenty? That's great to know. I've had many a story where readers have said, "This was so intense that I had to put it down on page twenty and then pick it up again four hours later." That's okay with me, normally, but if I have twenty readers and fifteen had that reaction, I know that the story probably needs to be toned down.
9) Gauge your writer's ability to take criticism. Some new authors might be brought to tears by a couple of honest criticisms. Always do your best to be both professional and honest. If a story just doesn't work, you need to let the author know that it doesn't work and why. Telling them is your job. Recognize that some people don't know how to take even the gentlest, most kindly intended of criticisms. Go ahead and be gentle, firm, and honest. If they never write again, that's their problem.
"Never demean yourself by talking back to a critic. . . Write those letters to the editor in your head, but don't put them on paper."…Truman Capote