If you have committed a crime or some other act of inappropriate behavior, you may be thinking about doing the honorable thing and confessing. Confessions help the police investigate crimes more quickly and may persuade a judge to be more lenient in sentencing. Be sure to seek legal advice before you write a confession letter. There will likely be consequences, since you're admitting that you're guilty of a crime.
TL;DR (Too Long; Didn't Read)
There's no specific confession letter format, but you must show remorse and be very clear about what you did, when, where and to whom. Without this information, there's no proof the incident took place.
When To Write a Confession
Do not write a letter confessing to a crime before speaking to a lawyer. However good your intentions are, you need to know what your exposure will be in terms of the criminal charges you are facing. There may be penalties beyond the crimes you are confessing to if you previously obstructed justice, for example. It's generally okay to write a confession letter in other situations, for example, confessing to an affair so your spouse can petition for divorce on the grounds of adultery. If you're unsure, take legal advice.
The Confession Must Be Voluntary
In most jurisdictions, your confession must be voluntary. This means that you are writing the letter of your own free will, and no one has coerced you into it, either through intimidation or with an offer of a plea bargain. The court will not accept your confession if it appears phony. Assuming you are genuinely remorseful and prepared to accept the consequences of your actions, start your letter with the statement, "I am making this confession of my own free will." It's probably not enough if there's clear evidence of coercion, but in the absence of such evidence, the courts will accept your letter as a genuine confession.
Be Specific About the Things You Did
A confession by itself is not enough for a conviction. There must be additional evidence – apart from your letter – that proves you committed a crime. In some states, the court will not even read your letter without this kind of corroboration. For your letter to be relevant, you need to specify what you did, when, where and to whom. There's no need to use legal language or write this information in a special way. Just be specific with the date and place of the incident, what happened and the identity of the victim. This will help the police to corroborate your story.
One reason for writing a confession letter is to show remorse and explain any mitigating factors so the judge will show leniency at sentencing. If that's the reason you're writing, include a clear statement of apology. Don't minimize the crime or attack the victim. Then, explain why you did what you did. For example, you might say that you stole food to feed your family or were going through a really difficult time in your personal life. The court usually will give you the opportunity to explain these mitigating circumstances prior to sentencing, but there's no harm putting them in a confession letter if you feel these things need to be said.