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The app has a blue icon with a white “F.” If it's installed, you'll find it on your home screen (iOS) or in the app drawer (Android). If you don't have the app, open a web browser (like Safari or Chrome) and navigate to https://www.facebook.com. Sign in with your username and password when prompted. You can do this by tapping Friends on your own profile, or by typing a friend's name into the Search box at the top of the screen. It's just beneath their profile photo.  A checkmark will appear next to “Restricted.” Now that your friend is on the Restricted list, they can only see posts you've marked as Public, and those in which they are tagged.  Adding your friend to a list will not send them a notification. To remove your friend from the Restricted list, return to Edit Friends Lists and tap Restricted.
Open the Facebook app. Go to your friend's profile. Tap Friends. Tap Edit Friend Lists. Select Restricted.