Very easy to navigate also. Once you pass the baggage return and exit the secure part of the airport, make a right turn and proceed to the end. There are a couple of machines there (look like ATMs) to purchase tickets for the next bus (every 15 min after the hour). I used a debit card for my purchase, as it didn't like the first credit card I tried. Once your purchase is complete, the machine prints out a small receipt which is your ticket.
The bus departs from just outside the doors where you buy the ticket. When I did it my bus was already parked there. Bus driver arrived a few minutes before departure and checked everyone's ticket as the boarded. It's open seating.
Edit: Bus only makes one stop (about 15 min outside of San Sebastián) and then proceeds into the city.