How to Work with iCloud Keychain
On your Mac, go to Apple Menu > System Prefs > iCloud. On the right, check Keychain’s box. If you haven’t enabled two-factor authentication to protect your Apple ID, then you’ll need to set a security code — don’t lose it!.
Connect a device
On a Mac that’s signed in to your iCloud account, tick the same Keychain box. On iOS, look in Settings > [your name] > iCloud. You may be asked to enter the passwords/codes for devices on which you’re already using keychain.
AutoFill in Safari
In Safari, go to Safari > Preferences and click the AutoFill tab. Here you can choose which information will automatically be filled out in forms. The first item pulls details from your card in Contacts, rather than using keychain.
The quick way to add usernames and passwords to your keychain is Safari > Preferences > Passwords, by entering them manually — be careful when typing! Click Add, enter a site address and your username and password for it.
Save at sign in
Instead of, or as well as, the previous step’s method, take advantage of Safari’s offer to save details when you first sign in to a site. Also, when creating a password for a new account, Safari suggests one; click it to accept it.
Save at setup
Alternatively, click the icon at the right of the field for another suggestion. Your credentials are saved to iCloud Keychain and are available on all devices you connect to it. Some sites — banks, say — can request for details not to be saved.
To change a password on your Mac, follow the site’s reset steps, then sign in; Safari asks to update your keychain. Or, preemptively click the password in Safari’s Passwords preferences to edit it directly.
When you visit the Passwords tab on a Mac, you’ll need to enter your password (or use Touch ID) to view items. Each is only readable when its row is selected; make sure you don’t leave this tab open and unlocked for all to read!
AutoFill on iOS
If Settings > Safari > AutoFill > Names and Passwords is on and you visit a sign-in page, saved credentials for the site are shown as pictured; tap one to fill out the form, or tap Passwords to choose from other saved account details.
Keychain on iOS
In iOS 11, apps can use keychain. If an app hasn’t been updated for this, go to Settings > Accounts & Passwords > App & Website Passwords, hold a finger on an item, pick Copy Username or Copy Password, switch to the app, then paste.
If you delete an account, remove its details from your keychain on one device and they’ll go from others, too. On a Mac, select it in Safari’s Passwords list and click Remove. On iOS, swipe left on it in the passwords list and tap Delete.
Add card details
In Safari > AutoFill, click Edit next to Credit Cards. Click Add and fill out the details. For security reasons, CVV codes aren’t stored; they must be memorized. To edit a detail, click it; altering the long card number requires your password.