Writing a Critique
Line by Line, Stanza by Stanza
A critique will be the most thorough broken down line by line or stanza by stanza, depending upon the construction of the original piece. This allows the commenter to focus more narrowly and spend time and thought on the details.
In-Depth
Go beyond "This is good" or "This is bad" and their synonyms; tell
why. Going line by line or stanza by stanza will help this; you'll be able to pinpoint what parts 'spoke' to you. Was it a particularly clever word play? Did that one image stand out so vividly you can't get it out of your mind? Likewise, you'll be able to focus on the real issues. Was the rhythm off in just that one line? Is that particular phrase just so cliché you couldn't read past it? Did that one word choice stick out like a sore thumb instead of like a prize-winner? Try to think of at least one positive thing to say, and one area for improvement. If nothing else, what was your favorite part of the piece - and your least favorite?
Take your time. Be willing to read, re-read, and read yet again. Make an honest effort to 'tease out' the meaning or intention the poet had when they wrote the piece and whatever specific part of it you're looking at. This lets you be more specific if nothing else; if you're way off base, they know the piece may not be communicating well. It also leads to more discussion, which is more helpful.
Ask Questions
Don't (necessarily)
think in terms of good or bad, but ask yourself, "What is this
doing?" Think about the effect the various devices and techniques employed in the piece have on its tone, mood, style, and message.
In the same way, a critique laden with questions (not the sarcastic kind, in this sense) may be an excellent one. It makes the recipient focus on the reasons for their choices -- and what those choices are doing -- and may lead to more constructive dialogue. If the writer can clearly explain their choices, they may well be valid.
Constructive = Works Towards Improvement
Be polite. "This is terrible" helps as little as a blanket and unjustified "This is the best poem ever!" Don't back away from telling the writer where the problems are, but simply point them out, perhaps with suggestions for improvement.