If you have already started a document, however, and wish to change it to landscape mode, you need to add a new master page. Select Add Page from the Page menu. In the dialog box make sure to select New Master Page. It will ask you to name the page. What you name it is not important, as long as it is not called left or right. You will then see a blank page. Choose Rotate from the Page menu.
Now, select Column Layout from the Page menu. Click on the Add/Change flow option, and make sure you have 1 text column selected. Then click on OK. You may need to resize the resulting text box.
Now that you have the master page setup, you need to apply this to any pages of your document which need to be in landscape mode. Select Body Pages from the Page menu, and the go to the page you want in landscape. Choose Column Layout from the Page menu, and apply the master page you just created (the first option has a pull down menu that lets you choose from all the available master pages), to the current page (or all pages, left pages, right pages). Then click on OK.
Now any time you want an additional page, you would again apply the master page you created in landscape mode to that particular body page.