A family of Microsoft word processing software products for creating web, email, and print documents.
The Letterhead detail needs to be inserted into the First Page Header of the document and any content for the second and subsequent pages needs to be inserted into the Primary Header
You should set up a template with those features and then use that template as the basis for creating new documents.
Start with a blank document and then right click in the top margin and then on Edit Header. Then set up the content for the Primary Header. After doing that, check on the Different First Page box in the Options section of the Header & Footer tab of the ribbon and set up the header content for the first page. Then close the Header and Footer and save the document as a template (.dotx)
Then, when you need to create a document with those features, use File>New and select that template.