From El Prat:
There are signs inside the terminal to the train platform. Buy a T10 ticket. (This gives you 10 journeys on all metros, buses and trams in central Barcelona.) From the airport there is only one train so you can't miss it. Get off at Passeig de Gracia station. You can either come out of the station and walk 4 blocks up to Passeig de Gracia 116. Or, change from the train to the metro (in the same station) Line 3/green line, and get off at the next stop, Diagonal.
The Aerobus goes directly from outside the terminal (signs in the terminal building) and it costs 5€ one way or 8.65€ return. Get off at Plaça Catalunya. Take the metro Line 3/green line from Plaça Catalunya to Diagonal. We're 2 minutes away from Diagonal metro stop. Come out, cross the street Diagonal and we're at Passeig de Gracia number 116.
From Girona or Reus airports:
Take the bus to Estació del Nord. From there take the metro from Arc de Triomf Line 1/red line to Plaça Catalunya. At Plaça Catalunya change to Line 3/green line to Diagonal. We're 2 minutes away from Diagonal metro stop. Come out, cross the street Diagonal and we're at Passeig de Gracia number 116.