Send the letter of apology immediately after the incident takes place. Allow no more than two days to expire before addressing the offense appropriately. The longer the offended person remains hurt, the harder it is to reconcile.
Explain the situation succinctly but do not omit the hard facts that caused the breech in the relationship. Don't try to explain away actions that were committed. Accept responsibility for what was done and refrain from blaming other people for the situation. Remember that the breech is between you and the person offended.
Tell the person in writing that you will not repeat the offense again. Explain how you regret the situation that took place and the pain they have experienced. Your words must paint a picture of a person who is repentant and serious about reconciling differences. Express how much you value the relationship and desire to better it over time. Demonstrate your commitment to the offended person in word and deed. Close the letter with "Sincerely."