Be Careful! The bus to Carmona doesn't leaves from the Prado de San Sebastian Bus Station but from a parallel street: Avenida de Portugal. Once there look for Bar Citroen and just in front you can take the buses to Carmona. Tickets can be purchased on the bus.
For more information consult the buses schedules on the official website.
Carmona is recommended by IndependenTrip as a day trip from Seville. For other day trips from Seville: Day trips from Seville