Book your tickets from Barcelona to Grenada via the RENFE Web site.
View the train travel options from Barcelona's Sants station to the train station in Grenada (located in Avenida de Andaluces off Avenida de la constitución). There are two routes; the Garcia Lorca, which departs three days per week, and the Talgo TrenHotel, which departs four days per week. Both offer comfortable accommodations and take about the same amount of time (11 to 13 hours).
Use the TIKNET system to make your booking. If this is your first time using the system, you will need to register first.
Print out the confirmation page and the confirmation email. Bring both documents with you when you pick up your tickets at the station.