Read the piece thoroughly. Before you begin to critique any element of a written work, you should first read through the entire copy to get an overall sense of its story, flow and theme. Read the piece first as an impartial reader, then ask yourself a few general questions from a reader's perspective. Did you find the piece interesting? Was it enjoyable? Informative? Once you have answered these questions you should go through and read the piece again, this time with a more critical eye.
Accentuate the positive. It is important that you hold a respectful attitude toward both the writer and the text during a critique. If you are performing a critique as a professional favor, you are not only providing assistance, you are also proving your worth as an editor. Begin your critique by listing the elements that worked well. Did the writer have a strong grasp of dialogue? Was the plot well-constructed? Was the pacing effective? Starting out with the positive information can help writers feel better about your critical assessments of their work.
Address certain specifics in the structure and tone of the piece. These often include characterization, dialogue, plot, continuity, theme and pacing. Make a checklist of each of these elements and go through the work to determine what works well and what should be improved. Does a certain character fall flat where another one shines? Is the dialogue wooden or believable? Does the plot hold the interest of the reader? It is also helpful to pose specific questions for the writer that will force a particular decision. For instance, you might ask if a character would really say a particular line or if a scene would be more effective if it was tightened.
Personalize the critique. Every writer is unique, and every written work is unique as well. Once you have analyzed a piece, you should ensure that it meets the tone and style that the writer was hoping to achieve. If he wanted the piece to be funny, does it hit that mark? Is it dramatic when it should be? Could it be more informative? Make sure you understand the goals of the writer so that you can better personalize your critique.