A key piece of adaptivity in a modern iOS app is supporting multitasking on iPad. By default, apps created in Xcode 7 and newer will be configured to support multitasking: they'll have a LaunchScreen.storyboard file that uses Auto Layout.
The easiest way for existing apps to opt in to multitasking is to create such a storyboard, then set it as the project's Launch Screen:
Once your app supports iPad multitasking, audit existing views and view controllers to make sure that they use Auto Layout and can support a variety of size class combinations.