Shake your device, hear the "Ding" sound plus a tiny skull will be displayed on the right top corner which indicates the game is in prank mode.
Shake your device again, hear the "Dong" sound and the skull is gone which indicates the game is returned to normal game mode.
The Setting screen is for prank mode. To see the setting screen, first shake your device to active the prank mode, then tap on the tiny skull on the right top corner and it will bring you to the setting screen.
First you need to create an account in the GameCenter. The GameCenter app is shipped with your device and just follow the steps to create an account. Once you signed in prior to play this game, the system shall remember your account, then when you choose to post your score, it will automatically bring you to the GameCenter. If you haven't signed in, in the game, a pop up dialogue will ask you to sign in first. After you signed in, you are free to post your score or check the stats from other players.
You need to get into the "setting" page( see the answer to another question). Once you are in the "setting" page, you can customize prank option to change "Disply Blood", "Use Different Image" to make it more real and fun for a prank.