Get some good quality chocolate, nothing less than 80% cocoa. Melt it slowly in a bain marie. (heat water over the stove with metal bowl on top, place choc inside, just until melted).
Then spread choc with a spatula on a clean smooth surface,start from the bottom and work your way up. You can get a special spatula at a baking shop.
Take a flat spatula and pry the chocolate away from the surface when it is just cold. Keep on rolling.