i need to make a cupola for a cake what is the best way to make it? 

On a balloon? I've seen this done with chocolate, RI or gumpaste scroll moulds. Obviously the balloon is only partially inflated and rested on a small dish while you work, then when dry the balloon is pricked. Don't forget to put shortening on the balloon prior to assembling your design.

Not on a balloon, it's on top of a building. 

what she means is that you make your cupola on a balloon :), then when it is dry/stable remove the balloon and put it in the building.

Oh my wee lamb....you've cheered me up x