How do I write a letter of apology to someone whom I have backbitten, slandered, and falsely accused, and what is the extent that must be mentioned in the letter to be absolved of these sins?

Scholars have differed concerning what is incumbent upon the backbiter. Some are of the opinion that it is necessary to seek absolution from the one who was backbitten. Sheikh al-Islam Ibn Taymiyyah holds that seeking forgiveness for him, mentioning him positively, and supplicating for him is sufficient. However, seeking his absolution—even in a general form—is better and more absolving of responsibility. The backbiter can inform the one he backbit that he has repented to Allah and apologize to him.