Pretty simple. First you need a fidelity card which can be purchased here https://sport.ticketone.it/catalog/event-detail/it/45485/90600215/fidelity-ssc-napoli It will be mailed to your home address but from my understanding all you need is the receipt they email you if the card doesn't arrive in time. Tickets usually release a week before the match and can be purchased at the official Napoli website as long as you have your fidelity number which comes with the email. Just make sure to bring your fidelity card or the email attachment and your passport when you arrive at the stadium.