You can either get the bus from the airport to Malaga bus station, which is around 1,00 Euro leaving every half hour and then take the 16.30 bus to Ronda from there, price 9,36 Euros, or you can take the Airport - Marbella direct bus at 15.45 (3,92 Euros), which gets you in to Marbella at 16.30 and then pick up the 18.00 Ronda bus (4,89 Euros - it's the same one that left Malaga bus station at 16.30). You will arrive at around 19.30 in Ronda. Both buses leave the airport from the same place. As you come out of the arrivals hall and see the people waiting to greet passengers, turn right instead of exiting in front of you and you will see the way out. Turn right again and the bus stop is only 50 metres or so from there.
Swings and roundabouts really. If you go into Malaga first you will arrive at Marbella via the towns of Torremolinos, Benalmadena and Fuengirola, one stop in each) and go along the coast road by the sea. If you go from the airport to Marbella, you go along the inland toll motorway. Both ways are equally interesting.
Huge saving to be made of about 1,50 Euros if you go to Marbella first! I think I would possibly be tempted to go that way if I had luggage because the bus to Malaga tends to fill up en route as it's not a direct service and could be a bit uncomfortable. The direct bus to Marbella is a coach with air con and reclining seats and a luggage hold, so you can just stick your bags in the hold then get on, but sit on the same side of the bus so you can keep an eye on them, just in case anyone takes a shine to them and decides to take them off again and pinch them! Never heard of it happening yet but best to be wary. You will have an hour and a half to kill in Marbella and there's not a lot around the bus station, but at least there are a couple of bars. If you wanted something to read, get it as you come out of the arrivals hall at the airport.
I think it will only stop at San Pedro after it leaves Marbella and the road after that towards Ronda is spectacular, so at Marbella put your bags on the left side of the bus in the luggage space and sit on that side for the best views, or in the front seat next to the driver if it's vacant.
Have a good trip!