"Bhikkhus, speech that is well spoken, not badly spoken, is blameless and worthy for the wise, has five characteristics. Which five?
It is spoken at the proper time. It is spoken truthfully. It is spoken gently. It is said in connection with the goal of Awakening. Speak with a heart [full] of loving kindness.
By these five factors is speech well spoken, not badly spoken, is blameless and worthy for the wise."
Anguttara Niakaya (V.198) My translation. See also Access to Insight
So when giving feedback we need to consider these five factors.
1. The proper time to speak
Pick your moment. Is it a good time for the other person? Is there time to follow up? Is this the right place? Public criticism is unlikely to be effective. Try to take into account the person's mental state - have they recently been getting a lot of criticism, have they been ill, had a bereavement? Are you in a good state? Is the other person ready to hear what you have to say? Sometimes we need to be prepared to wait.
Stick to the facts, be straight forward. Do you know the facts? Stick to things that can be observed - don't make assumptions, and especially try to not second guess what someone else is thinking. Do no exagerate or minimise. Watch for your own biases - if you are angry you are unlikely to be able to do this for instance. It may mean that rather than criticising someone else when you are angry, that you apply the criteria to yourself and confess your anger instead!
Truth has a particular power in the Buddhist tradition. There are several stories where the speaking aloud of a simple truth, the sacca-kiriya or Act of Truth, changes the course of events. Angulimala, for instance, eases a woman's painful childbirth by stating that since his noble birth, he has not intentionally killed any living being. There are many Jataka stories with a similar theme.
Always keep in mind that the truth can be painful to hear, and that you might not posess the truth!
3. Softly and Gently
Speak gently and kindly. Be respectful. Calm, reasonable and clear. If you can't do this then reconsider giving feedback. Never swear or use harsh language. The reason for this is that if you start to yell and use harsh speech then you will almost certainly cause the other person to be defensive. They will rightly fear for their safety, because you are allowing your anger to control you. If your anger is controlling you then there is no telling what you might do!
Any drama that you bring to the situation will detract from the message that you are trying to get across. But recall that we are being truthful, so there is no need to go soft on the issue.
4. For the purpose of Awakening
Your goal in giving feedback should be the highest goal. If you are simply indulging your likes and dislikes then that is quite ignoble, according to the Buddha. You might ask yourself, who will benefit if I get the change of behaviour that I am seeking? Is it only me? How will the other person benefit if I tell them what I wish to tell them. In fact if you are simply trying to get someone to conform to your will, then you are doing them a violence.
If you are seeking a change in behaviour then offer a towards (desirable behaviour) and an away (undesirable behaviour) when asking for a change in behaviour. Just telling someone to stop doing something is less likely to work than asking them to start doing something else. But remember that they must be free to decline your request. Coercion of any kind will not conduce to the good.
Are you the right person to give the feedback - maybe it would be better coming from someone else?
5. From a loving heart
Your every word should come from Loving Kindness. This is not restricted to criticism of course but goes much much deeper. In fact you could almost say that if you are not communicating metta then better to do as Shantideva suggests and act like a block of wood. It may be that you need to hold back on giving your feedback until you have had a chance to develop a loving heart towards the other person, perhaps by putting them in the fourth stage of your mettabhavana.
So these are the five factors to consider. There will be times, of course, when it seems unwise to pause to reflect on all of these factors - if our friend is about to step into the road not having seen an approaching car for instance. In that case, of course, we simply act. But in most cases there is time to reflect, and it will be useful to do so.
Making connections with other people is vitally important to our well-being, but also to our Buddhist practice. Bungled feedback is a common way in which we fail to make connections, or we attenuate or even sever, the connections we already have. Repairing rifts, and restoring harmony in the Sangha takes much more time and energy than maintaining good communication.