@JohnB2
Good explanation!

I will simply add that the letter should be focused on what you did wrong, not WHY you want to go. Also, contrition, contrition contrition! You were found guilty, so defending yourself in this letter is a mistake.

I never let the client write his own letter. Clients always defend themselves and mitigate the situation. My goal is "what does Homeland Security want to hear to say yes?"

I'm no expert by any means but I can share some suggestions. Don't admit to any more wrong doing than what they already know about. If you were denied crossing the border, whatever they told you the reason is is what you would want to focus on.

I would suggest explaining what happened (what you did wrong) and include any circumstances that at the time may have contributed to why you did what you did. By that I mean, unusual circumstances that someone could look at and say, "oh, yeah. I can see that" and they could also see that circumstances today are different and thus the issue wouldn't likely happen again. But it can't sound like an excuse. Its important to own up to whatever happened.

They are also going to want to see evidence of anything that has changed that would assure them you're a different person today and learned from the mistake. If you did community service, for example, or took some treatment program...those kinds of things are helpful.

They want to see strong ties to your country and that you won't likely come to the U.S. and stay and not return to your country.

They also will look for what the benefit is to the U.S. if they allow you to enter. E.g. if you can say you're coming to the U.S. to console a friend because someone died or your volunteering for something that helps people in the U.S. You get the drift.