Buy tickets for the Arian Baptistry in Ravenna from the ticket machine, online, or in person from the Ravenna National Museum — not from the ticket office of the main mosaic sites. The ticket machine outside the Arian Baptistery resembles a car parking ticket machine. English instructions are stuck to the machine, but the attendants frequently help visitors (and probably also think simply collecting the €2 coin at entry would be simpler for all).