Critiquing is a great way to improve your writing. Exchanging critiques is also a good way to get free input on your manuscript, and you all know what I think about having a critique partner. Not to mention, a lot of unpublished writers are REALLY good and it's an opportunity for free reads some of which are amazing.
Bottom line is if you're critiquing someone's work, most reasons you'll have for doing it are good ones. But the critiques need to be good too, and good intentions don't always translate into good suggestions.
So how do you critique? What makes a suggestion a good one?
I don't think there is any right way to critique/edit. I think most of the time our critique methods will be as different as our writing styles. But some basic standards can/shoud apply to all of us.
You should start out with an agreement of how thorough critiques need to be and a timeline for returning work. Clear expectations are easier to meet, which is better for everyone involved. You can avoid frustration on both ends this way.
But what about the actual comments? What you actually say when you critique someone's work is what I want to talk about today. You need to be as specific as possible. Don't use a phrase or expression you assume is common knowledge. There is a good chance it might be, but there is also a chance it's not. And even if it is what if the person you're critiquing for is from a different region of the country/world where that expression is not used? Or hasn't been writing very long so while "show don't tell" is ad nauseum to lots of writers it means nothing to them? So instead of "show don't tell," try "I need to know what the quilt looks like." An agent once left a comment on my query, "I don't care about any of these people." I read it and thought she just wasn't a nice person, because I had heard that there were things in my book that needed to be fixed, but it was pretty much a consensus that characterization is my strong point. A month later at a conference, I put my hook into a critique basket. The commentor read it and said, "This doesn't tell me anything about the MC. There has to be more to her than this."--That made sense. I suddenly understood what the agent meant when she said "she didn't care about my characters." They were developed in the book, but i hadn't described them in the query.
Ask questions. Sometimes asking Why? or How does the MC do this when X event is happening. Or any number of questions is more helpful than a comment. It lets the writer know what's missing, or what you're looking for. But sometimes you can see something is missing the writing and don't know how to specify what. Asking a question helps lead the writer to that detail of what exactly is wrong.
In a nutshell, be as clear as possible. Critiquing is hard work on both ends. It's a tough balancing act to have to point out everything that's wrong with someone's work product without feeling like a bully. And opening a document full of red ink is no fun either, but that writer would rather see it from a friend or partner than an agent/editor. Since it's such a painful process, it should be as productive as possible. Ambiguity doesn't help anyone. And if you think you've been clear, worded the comment as precise as words will allow, asked all the right questions and aren't seeing a change, you might find a different way to say it.