A personal apology letter should contain an admission of responsibility and a promise not to repeat the mistake or offense. It should also show contrition, ask for forgiveness, and, if appropriate, include recompense.
An apology is a statement of regret and sorrow for an offense against another. An apology letter is simply a written apology in the form of a letter. It does not need to be lengthy, but should clearly articulate the nature of the offense and explain why it occurred. Generally, an apology letter is not necessary for a minor faux pas, but rather for something major. The determination of what constitutes “major” should be with reference to the offended party, not the offender.
Before starting a letter, the writer should consider whether writing and sending one is the best approach available. There are other ways to acknowledge a mistake, including ignoring it, verbally apologizing, sending an "I'm sorry" card with little or no elaboration or meeting with the offended party face-to-face. Unless a letter accomplishes more than these alternatives, there is little need to send one.
Sometimes, writing a letter can be useful even if the writer does not intend to send it. This is because the writing process forces the writer to reflect and helps one adjust to the realities and consequences of what has been said or done. Generally, the worse the mistake, the greater the benefit there is to doing this.
Finally, the writer should refrain from sending a letter if there are concerns about legal consequences. In some cases, an apology letter could amount to a confession of guilt or an acceptance of liability in a court of law.