So your charge is to be honest without being brutal. Your comments are more likely to be heard if you make suggestions rather than give orders. Refer to specific places in the text wherever possible, and be courteous and professional at all times. Even a story that needs work has its eloquent passages, its memorable characters, its ingenious plot twists. Look for them, and point them out when you find them.
People do their best work when they re feeling confident, so don t stint on the praise.
Remember that a good critique focuses on the work, not the writer. Criticism shouldn t be given, or taken, personally. Before you submit your critique, read it over and ask yourself how you d feel if your remarks were posted about something you d written. Although what you re reading is only an excerpt from a longer work, you should be able to make well-reasoned judgments about the work as a whole. Publishing professionals often make decisions on far fewer than fifteen pages.