Once you have found the skull in campaign, (Normal difficulty or above,) hit the X button in the campaign lobby. Select the skull you would like to activate and hit A.
Once you have found the skull in campaign, (Normal difficulty or above,) hit the X button in the campaign lobby. Select the skull you would like to activate and hit A.
Once you have found the skull in campaign, (Normal difficulty or above,) hit the X button in the campaign lobby. Select the skull you would like to activate and hit A.