Accentuate the positive
If you are going to damn a person, then you need to handle the situation very carefully. Your future relationship is at risk if you do it clumsily. Trust lost is hard to regain.
The most important rules of classroom observation must never be broken. Number one, give feedback quickly. Don't let days go by. Any long delay will cause a build up of tension and potential bad feeling. The more difficult the feedback, the more important it is to do it quickly.
Second, always start with what went well in the lesson. Pick out all the positives, notice what the teacher has done that is good. Then, and only then, pick up on the aspects of the lesson that could be improved.
My experience is that when people are feeling vulnerable during a feedback session, they will accept difficult news far better if they feel confident that you have really noticed the detail of the good things they did. It is basic human psychology. If a person feels you only see the bad in them and never the good, they will be inclined to dismiss all your observations and advice, and label you unfair.
In the many lessons I have seen, there have been some truly terrible ones. But even in these, there has usually been something that was going right. And that's where you start your feedback.
Paul Blum, A manager in a London secondary school.