Sorry letter is written to say apologies for what you have done to the reader. The letter should be written with genuine feeling and should not be fake.
Sorry letter can be written for formal or personal purpose. The reason for saying sorry should be mentioned and if there is any need to clarify anything that you want should be mention in the letter.
It is best if sorry letters do not tend to justify the cause of grievance or the hurtful action. This defeats the point of the sorry letter which is to offer an unconditional apology. Hence, one must admit one’s fault and not be defensive while apologizing. This result is a convincing sorry letter.