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 โฌ2 coin at entry would be simpler for all).