Go to the side of the building where the hanging window cleaning thing is, then stand back and run run towards the edge and jump at the thing.

Then press up and you will climb up it like a normal ladder, then you climb on to the ledge on the side of the building and walk round the building until you come to another window cleaning thing and then you climb.

Then walk up to the top of it, then climb up onto the bar at the top of the building and walk along it and there you go - you're on top of the building!

Be patient cause it took me a couple of times to perfect it!