Here is a primer from  Nolo.com on this very topic:

“The demand letter is the centerpiece of the insurance claim negotiation process. In it, you set out to the insurance company your strongest arguments concerning:

• what your injuries were and are
• why the other person is legally responsible for your injuries
• what your medical treatment was and how much it cost
• what your income loss was
• what other damages you suffered.

Your letter should conclude with a demand on the insurance company for a lump sum to settle your entire claim.”