Be sure to make a copy of your signed letter, too, in case you need to reference it in the future. Once your insurer receives your letter, you should receive a USPS green card as confirmation, which you’ll want to keep with your letter.
Depending on your situation, you may want to reject the first settlement offer. Since insurance companies lose money each time they pay for a claim, your first offer may not be as high as it should be. If you find yourself in this situation, you may want to work with a good lawyer to navigate the legal intricacies.