From JFK Airport:

Go North on 134th to S. Conduit Ave,
Stay to the Right onto the ramp, turn Left at 150th.
Go Left to North Conduit Ave to the Vanwyck Expressway via the ramp on the Left.
Take Exit 12B to 495W and keep left to the Midtown Tunnel.
After the Tunnel, make a Right onto 34th Street
Left to 9th Ave 
Left on Hudson
Right onto Gansevoort
Right onto 12th Street and a final Right onto 9th Ave.

From La Guardia Airport:

Take Parkway West onto the Grand Central Parkway
Exit 4 to the Verrazano Bridge
Go to the Brooklyn/Queens Expressway East. 
Take Exit 35 to 495W toward the Midtown Tunnel. 
After the Tunnel, make a Right onto 34th Street
Left to 9th Ave 
Left on Hudson
Right onto Gansevoort
Right onto 12th Street and a final Right onto 9th Ave.