If you want to start with a letter, consider paying an attorney to write one for you. You can probably expect to pay $100 or less, unless the situation is complex and it takes time to explain it to the attorney.
The letter should say something like "I've been asked to contact you...." It cannot say "I've been retained by..." because that's not true. You're just paying the lawyer to write the letter; you're not hiring him or her—at least not yet. However, the attorney's letterhead serves to show the debtor that you are serious.