Welcome to the forums!
I'd suggest reading the following:
Common Abbreviations
Credit Scoring 101 - great for knowing what is in your credit score and to see how your score is impacted.
What Steps Do I Take - great for learning the repair process.
and Example letters - PFDs, GWs, DVs, etc.
If paid, you'd send a GW letter to whomever is reporting. If a CA is reporting it, then an OC could be too. In that case, you'd send GW letters to both. Just tailor it to suite your experience. Mention a life event, job loss, medical issue, etc. I used my "stupid credit phase" as an excuse. YMMV based on your own situation. But keep it short, and brief, and be cordial.
Persitence does pay off.
I'm mostly out of GW-sending, but when I did, I would start with one letter and I'd address it "Dear Sir or Madam" and I'd send it to the exact address as being reported on my CRs. If you get your CRs directly from each CRA you'll see detailed address info. After 30-45 days, I would send another. If that failed, then after 30 days, I would send it to that first address, but then I'd add it to all the addresses found on their website, their privacy statement, their contact us section, their SEC filing info if public, and to any address listed in correspondence to me. If after 30 days that didn't work, I would send more to those same addresses and I would add to the execs of that business. That info can be found via their website, via the Consumerist, via Jigsaw, SEC filing info, etc. If after 30 days that didn't work, I'd send letters to all of the above and then I'd start calling them for e-mail info, fax, and other info. If that didn't work, then I'd keep after them.
Give them 30 days or until you receive a response. Some OCs/CAs only update once per month. Also, I suggest subscribing to a CMS that pulls all 3, like CCT. That way if changes do happen, you can see that change on a daily basis so you are no sending GW letters to a baddie that was already fixed.
Finally, be careful with OC accounts. Closed OC accounts, even if bad, can still help your length of history and your AAoA. Some may opt to delete vs. removing a late. You really don't want to be in a position where a GW removed a baddie and your score dropped because it was one of your oldest accounts.
