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One of the simplest methods is to cut artist's board into the shape desired, making each form close to concentric and smaller then the one below it. 
one simple way is to make a simple electric circuit 
material are battery, resistor, key, wires and ammeter 
Another way 
I would use paper mache. (See below for a link to paper mache technique.) Construct a form out of chicken wire or something else pliable, then apply the paper mache strips to it. Wait till dry, then paint to look like a terraced farm field.......!!
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