In my creative writing class, there were only three people who were majors and knew how to write a well-rounded story. So I got my fair share of stories that didn't have proper syntax, or grammar, and were just hard to read.
So I perfected this "shit sandwich" to critique their writing; it wasn't until I discussed my strategy with others that I realized they did this too.
To make a great "shit sandwich"
Start with at least three good/great things about their story. For instance; "As I read your story, the details were really showing me where I was, how your character was feeling, instead of telling me (***author's note: going into depth helps); You really made me feel like I was your character, feeling how they did (***again, depth is key)...." Et cetera. You find three or so things that you really liked about the story.
Next, pick a couple things that they can work on. "Your main character and their boyfriend are supposed to be in love, but they feel detached from each other; their actions speak differently than how we're supposed to think they feel." Or, "Your continued use adjectives (the fact that you have twelve adjectives) within the first fifteen words of the first line really bogs down exactly what you're trying to get across. You can clean this up by choosing two or three adjectives." Et cetera.
Finally, wrap up with three things you liked about their story (again). Just like the first section of your critique, you'll want things that were good. You'll want to choose things that you haven't said before, to keep it fresh.
And that's all you really need to make a "shit sandwich." Top layer, good things; middle layer, things that need improvement; bottom layer, good things.
You may find that there are only three good/great things, so you'll want to determine how much goes into each layer.
Another critique that I use, if there's not a lot of good things for you to reflect upon is an A-B style.
I usually start my A section with the bad critique. I'll fill this paragraph with three things that need work in the story. Sometimes, I would find more things bad than good with the story (if it's a person didn't care about the story, or had no understanding of how to write). So I would only pick a couple things that needed improvement.
Then, the B section is your good critique. I always feel like it's better to end on a high, than a low. Sometimes, depending on the situation, I would reverse it so I did my bad critique first, and then good. Really, it all depends on the person I'm dealing with. Some people I knew would need to be "buttered up" before they were okay hearing bad news about their stories. Sometimes, a person could deal better hearing the bad news, but then reaffirming that they did something good.
Anyway, I'm just curious as to what you guys do.
TL;DR: What's your preferred way to critique someone? Do you go for an A-B format; or do you go with a "shit sandwich" format, like I like to do?