How do I choose which page uses which Master?

When you create a new page you can choose which master you would like the page to be based on. If you want to change its master after the page has been created, you can click on the little arrow at the top right hand corner of the Pages palette and choose Apply Master to Pages. Again, if you cannot find the arrow, right click on the bottom portion of the palette, where the pages are displayed.
Just specify the master you would like to apply, and type in the pages to which you would like it applied, using commas to separate numbers. Then press OK and the master will have been applied. You can tell because the prefix letter (default is ‘A’) will appear on the little page in the page listing window.