An apology letter should be sincere and assume full responsibility for the letter writer's actions. Whether the writer is writing a personal or business apology letter, it may be a good idea to write it out by hand for a more personal feel.
It is important to start an apology letter by saying that the letter writer is sorry. This should be sincere and should not come along with excuses for what happened.
Next, it is important for the writer to take full responsibility for his or her actions. Especially in a business apology letter, it may be the case that the letter writer did not personally cause the problem; however, it is important not to assign blame to someone else.
It is also crucial for the letter writer to put himself or herself in the recipient's shoes and try to imagine what the recipient felt and experienced as a result of the writer's actions. Recognizing why he or she would be upset and writing it out on paper makes the recipient of the letter feel heard and understood.
If the letter is from a business, the next step is to guarantee that the mistake will not happen again. The letter writer should outline the steps he or she has taken to personally ensure that the letter recipient will not have the same experience again.
Lastly, it is a good idea for the letter writer to once again emphasize that he or she is sorry. As a closing line, it may be a good idea to extend an invitation for an in-person meeting to rebuild a friendship or business relationship as applicable.