ON IOS 10.2 OR EARLIER
In Settings > iCloud, tap your Apple ID followed by Password & Security. Tap turn on Two-Factor Authentication followed by Continue. You may be asked to answer your Apple ID security questions to proceed further with this.
In Settings > iCloud, tap your Apple ID followed by Password & Security. Tap turn on Two-Factor Authentication followed by Continue. You may be asked to answer your Apple ID security questions to proceed further with this.