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It's a gray app that contains gears (⚙️) and is typically located on your home screen. It's the section at the top of the screen that contains your name and image if you've added one.  If you are not signed in, tap Sign in to (your device), enter your Apple ID and password, then tap Sign In. If you are running an older version of iOS, you may not need to do this step. It's in the second section of the menu. It's near the bottom of the "APPS USING ICLOUD" section. It will turn green. If prompted, enter your Apple ID and password. You can choose to use your device passcode as your iCloud Security code by tapping Use Passcode, or create a new code by tapping Create Different Code. If you created a new code, enter it in again to confirm. This is for security verification purposes. It's in the top-right corner.  iCloud Keychain is now enabled. Your passwords and payment information will be encrypted and available on all iCloud-enabled devices that you've signed into with your Apple ID. Any previously saved usernames and passwords, wireless network information, and internet accounts are automatically updated in iCloud Keychain when you enable it. Do so by tapping iCloud in the upper-left corner, then Apple ID and Settings in the same location. It's near the middle of the menu, next to a blue compass icon. It's in the "GENERAL" section. Doing so enables iCloud Keychain to automatically fill in your username and password when using Safari. Doing so enables iCloud Keychain to automatically fill in credit card information when using Safari. It's at the bottom of the menu. Your list of saved credit cards will appear on the screen. Tap Add Credit Card and enter your card information if you'd like to add a new card; tap Done when finished.
Open Settings. Tap your Apple ID. Tap on iCloud. Tap Keychain. Slide iCloud Keychain to the "On" position. Choose your passcode settings. Enter your security code. Enter a phone number that can receive SMS messages. Tap Next. Enter your Apple ID password. Tap OK. Return to the Settings menu. Scroll down and tap Safari. Tap AutoFill. Slide "Names and Passwords" to the "On" position. Slide "Credit Cards" to the "On" position. Tap Saved Credit Cards. Enter your passcode or touch your finger to the Home button, if Touch ID is enabled.