Here's the timetable - scroll down to find Ronda.
Often they don't open the online bookings until a few days before departure. But you don't need to buy in advance online. You can get the ticket the day before in person from the bus station, if you feel insecure about it, or just turn up early on the day to give yourself time to queue and buy the ticket.
Here are two suggestions for things to do:
Have a great time!
I checked the site and saw the schedule, but couldn't figure out how to buy the tickets.
I didn't see the second post.
I'll check it now.