This is a very good question. Thank you.
Start by finding your owner policy. You likely received it by mail with the original recorded deed.
If you can't find your policy, you'll want some evidence that you purchased an owner policy. A copy of the title insurance commitment along with the HUD-1 Settlement Statement showing a premium paid for owner coverage will suffice.
The policy or commitment will give you the name of the title company. You are looking for the big underwriter's name, like Chicago Title or First American Title. If you are lucky, the policy will also give you their address. If not, Google it.
Your letter should be addressed to the title company with a copy to your title agent or attorney, whoever performed the transaction for you. Send the letter to the attention of the Claims Department and the salutation To Whom It May Concern.
Make reference to your owner policy number and property address.
Be very detailed in the description of the claim. You don't have to write a novel but don't skip over important facts.
Include good contact information for you including address, phone numbers, fax, if any, and your e-mail address.
Be sure to sign the letter.
Include a oopy of the policy or if you can't find it, the other evidence. Also include any correspondence related to the claim. For instance, if you have received a tax bill or some demand for payment, include that.
As I mentioned before, a full copy should be sent to the title agent or attorney who handled your transaction.
It's not a bad idea to send this letter registered mail.
One more thing, if the matter causing you to make a claim has a time deadline, for instance a tax sale or court date, don't procrastinate. You have an obligation to mitigate damages and give the title company an opportunity to defend the title. I'd recommend calling them AND sending the letter.