How Do You Address An Envelope For A Widow?

The old-fashioned style, whether widowed or not, is to use Mrs. John Smith---the married title, then the husband's first and last name. A more modern version is to use "Mrs. Jane Smith"--the married title, her first name and the married last name.

Ms. Is used for adult women who have not yet married or you are unsure of their status.
Miss is reserved for girls and young ladies (but rarely beyond college).