The book lays out 10 guidelines for observer feedback. The first one is to avoid generalizations. This means that when you compliment or critique someone, be specific about what they did well, or what they could have done better. Just saying "good job" is too vague and doesn't necessarily help the person understand what they did well.
The second guideline is to focus on behaviors that can be changed. For example, if a member of a group takes on a self-centered role and starts hurting the group, the observer and the group will want to address that and make sure that everyone is behaving in the best interest of the group.
The third guideline is do not debate or argue with the group you are evaluating. Since the observer is supposed to provide helpful feedback to allow the group to improve, it would be counterproductive to argue. Getting input from the observer can be helpful, but the group members are the ones that ultimately decide if they accept the observer's opinions. If the observer persists and clashes the group, it will probably lead to less productivity.
The fourth guideline is to not micromanage the group process. Observers are supposed to help a group improve in specific areas, not try to control every aspect of the process. As the book alludes to, it's better to  focus thoroughly on a couple things and do a really good job rather than trying to cover everything.
The fifth guideline is to remember your role. The observer's main purpose should mainly help a group evolve and facilitate a productive environment. That's why it's important to give positive feedback as well as constructive criticism. Negative feedback will only discourage the group participants and won't necessarily help them if the feedback is vague.
The sixth guideline is to accentuate the positive aspects. Whenever someone gives me feedback, it always helps to hear them say something positive first because it builds confidence and gives me a sense that I'm going in the right direction.
Seventh is to end with specific suggestions for improvement. It would be very unhelpful if an observer told a group all the things that they didn't do well, but didn't provide any suggestions to help the process; that would be just as good as wandering around in the dark without a flashlight. By giving specific suggestions, the group can try them out and continue to develop instead of remaining stagnant.
The eighth guideline is to be clear and concise if you present your ideas orally in writing. If the observer writes whole paragraphs at a time, that can be overwhelming for the group members and may discourage them if it's all about the stuff they need to improve. Orally, it's best to be concise because it's easier to remember what the observer said if he or she used less words. Also, long winded ideas can get boring and cause the group to tune you out.
The ninth guideline is to compliment in public and criticize in private. This is something that I've learned about since I started coaching my hockey team. As a coach, you'd rather have a one on one meeting with a player if you're going to criticize him/her rather than chewing that player out in front of the entire team.
The tenth guideline is develop positive relationships during the feedback process. Since the observer's job is to help the group, they want to maintain a good relationship with the group by not blaming people or giving negative criticism.