Please, I was disfellowshiped. I want to write an apology letter regarding my sins of fornication, pride, and disobedience.
There is no requirement in the Bible for a formal letter to be written in order to apologize for your sins. What is needed is an acknowledgment that the sinner was wrong and that he has decided to change (II Corinthians 7:10-11).
While a letter is not required, if you want to send a letter, then it would be best to include those two elements: an admission that you were wrong and a statement that you are changing. For example, Zacchaeus was a tax collector who made his money from overcharging people. He declared that he was going to give half his money to the poor and if anyone is able to prove that he overcharged, he would pay it back four times (Luke 19:7-8).
Thank you for the clarification.