Go up the stairs to the helicopter, turn right and you will see a platform - jump onto it.

Walk straight across and you will see another, jump on that one, take a run and jump onto the roof of the elevator/lift/

Take a run and jump onto the ledge, follow the ledge until you see a crane, jump on it and turn right until you see a ledge on the wall.

Jump on it and follow it until you come to a cleaning thing.

Jump in it, follow it and come to the end of it, jump back on the ledge and then follow it until you reach the corner.

Stay as close to the wall as possible and walk along the ledge until you come to the window.

Crouch or go prone through the window and there you have a great sniping spot!