The Screen Layout Editor (arandr on the command line) is a graphical tool for selecting display modes and setting up multiple displays.
You can find this tool in the Preferences menu, but only if the 3D graphics driver is being used, as it is this driver that provides the required mode setting functionality.
On the whole, the editor is self-explanatory: use the Configure menu option to select the screen, resolution, and orientation. If you're using a multi-screen setup, drag around the displays to any position you want. When you have the required setup, click the Tick button to apply the settings.