You go on the map Venice and walk to where there is a slightly raised platform and a wall to the left.

You won't be able to jump on the wall, so you go on the slightly raised platform (be careful not to fall down the hole!) and jump onto the wall.

There should be a green umbrella thing.

You jump on that and then jump onto the wooden bit.

Walk straight across turn right and jump onto the fence.

This is the most difficult bit - you jump (not too far!) straight ahead.

You should land on a little part of the fence, jump the other side.

Turn right and go forward until you come across a part of the roof.

Hop onto the fence and jump on it and you are on the rooftops!