The goal is to help another writer improve. Keep it impersonal - avoid any form of nastiness. Give the type of feedback you would like to receive. Give praise where praise is due. Do not be tempted to rewrite for the writer.
Examples are sometimes appropriate.
They can be adjusted as needed. Have copies of your work for each person so that we can take them home for the week and comment on when we return to the group. Most of us prefer you to email us the work. Each author has the choice of having have a discussion on the piece at the next meeting or just taking the comments home to consider. This will help readers offer better quality comments because they can read chapters with context awareness.