Read the material thoroughly, then reread making notes of anything that catches your attention, confuses you or raises questions.
Write a summary of what the work is, in simple objective terms.
Make two lists: areas that seem strong or effective about the writing and areas tgat are problematic. Assess the opening strong, the clarity and the transitions? Be specific, noting paragraphs or page numbers.
Avoid inflammatory language. Don't use words that seem mean-spirited or negative. Such words include "never," "always," "terrible," "unprofessional" and "ridiculous."
Keep your personal feelings out of the critique. Don't argue with an opinion that you don't agree with. Point out weakness in the arguments or in how the writer presents that argument. Similarly, if a character is a type of person that offends you, remember to separate that from the writing. Don't write a diatribe against anti-feminists because the lead character is a misogynist.
Maintain a balance of negatives and positives. No matter how troubled a piece of writing might be, build on the positives. Give concrete and specific tips to help the writer grow. Try always to end the critique on an upbeat note.