You have certainly noticed that nothing has changed in your document yet. So why did we create master pages if it doesn't change anything? Simply because these are useful when they are applied. And that Scribus has no means to understand where we want to apply them. So we'll need to do it ourselves.
You may already know what we'll say: you have two ways to apply master pages, one using the Arrange Pages window, and another using a menu.
You can apply master page to individual pages. You can do it:
When you create a new page
When you want to apply a new master page to an existing page
If you want to create a new page base on your new master page, refer to the Adding pages one by one section of this chapter. You may remember that dragging a master page on a deposit area will create a new page based on this master page.
So what about modifying an existing page? Perform the same process, but release the mouse when it is above the page you want...