From the airport
Taxi: It's a 20-30 minutes trip and is around €30.
Aerobus shuttle: Drops off at Atocha, Cibeles (night) and O'Donell-Doctor Esquerdo to Airport T1,T2 and T4 and costs 5€.
By metro, catch Line 8 from Barajas to Mar de Cristal. At Mar de Cristal, switch to Line 4 and get off at the Serrano stop. This trip will take about 45 minutes.