Read your peer's draft. As you read, use your pen to make comments in the margins or underline points where you are confused. Be sure to read the entire draft before you make any judgments about it.
Think about your peer's draft. For an essay, does the argument have a logical flow? For creative writing, is the plot creative or cliche?
Start writing or typing your critique. Point out the good parts of the draft first. Always start with the parts your peer did well on. If she used outstanding evidence, mention that. If her imagery or character development is effective, tell her.
Use discretion when commenting about negative aspects of your peer's work. Try not to comment on stylistic or word choices unless the work is hard to understand. Remember that your peer has his own style just like you have your own style. Both can be effective.
Explain why you made the comments you did. Say why you think certain things are effective and why others are not. Encourage her to use more of the effective techniques you point out. Make suggestions to your peer about how she can improve or fix the weaknesses of her draft. She may not have thought of something that you have.