When you create a new document, Scribus automatically creates one or more master pages: one for a single-page document, two for a double-sided, and three for three-folded and four-folded documents.
Create a new Letter, 8 pages double-sided document, with Magazine margins.
Go to Windows | Arrange Pages and verify that you have two master pages, Normal Left and Normal Right.
Double-click on the Normal Left page from the Arrange Pages top part: Scribus will show the content of the master page instead of the current page.
See that this master page has margins. A quick glance at Page | Manage Page Properties will show that Magazine margins are used as defined in the New Document window.
Change the margins to Golden Mean and validate.
Add a rectangle at the top of your master page and set it nicely with Properties Palette.
Close the Edit Master Pages window by pressing the Esc key or clicking on the close button in the header bar.
You should see your document...