Go to the last page you want to keep vertical (Portrait) then select Page Layout | Breaks | Next Page.
Now click anywhere in the page where you want to have the horizontal page (Landscape) and go to Page Layout | Orientation and select Landscape
and doDo the same steps for the following page, the one after the landscaped one, but this time choose Portrait / ONLY DO THIS IFPortrait. Only do this if you want to restore the Portrait orientation back to normal.