(And correcting hfly on a couple of points - as he says, he’s never taken the bus!)

head to the far RIGHT and down to -2 to get the bus, if you walk fast it’s only two minutes from leaving customs (made the bus by 30 seconds) and they take card or cash but not istanbulkart. It was 75 minutes to Besiktas, so about 2 hours to Kadikoy including the ferry, slower than a taxi but not massively and made a nice change… took the same route in reverse today and without the queue yesterday afternoon behind people trying to get over the bridge it was only 50 minutes from Besiktas to the airport.