How do I sign into Game Center? (iOS, any app)

Please follow the screenshots below to sign into Game Center. Note that after doing so, you may have to fully exit and then relaunch your game before the game will recognize that you are now signed in. You may also have to connect your game to Game Center. But first do the steps below to sign in.

iOS 7 and above

1. Launch your Settings app.
   
2. Scroll around and look for "Game Center".
   
3. When you find "Game Center", click it.
   
4. Enter your Apple ID (it's an email address) and your password.
   
5. Click "Sign In".
   
6. Your screen should look something like this if the sign-in succeeds. Notice that there is no longer a Sign In option because you're now signed in!
   

Below iOS 7

1. Launch your Game Center app.
   
2. You should arrive on a Welcome page, unless you're already signed in.
   
3. Enter your Apple ID (it's an email address) and your password.
   
4. Click the "Sign In" button.
   
5. Confirm that you're now signed in.