If you are using letterhead paper, use it only for the first page. All subsequent pages should be plain paper.
Near the top left corner of the second page, type the addressee's name, the date of the letter, and then "page 2". Example:
Mr. Russell Mael
November 14, 2006
Go down about four spaces or so and then continued your letter from page one.
Use the same format on page 3, 4, etc.