When you arrive at South Terminal, proceed up one level to the departure level and head to the drop off area. A bus stops a bit past the furthest door to your right as you exit. I don't think it's marked at all but it was an old, small (~25 seater) Japanese thing when I last did the transfer. There is usually someone who knows what's going on if you ask some of the guys hanging around looking like they're waiting for a bus.