Do not unwrap the chocolate, but hit the bar hard on the work surface several times (change the place where you are holding it each time). The bar breaks into small pieces in the paper. After that all you need to do is open the wrapping over a bowl, and drop the pieces in.

Be careful how you choose the quality of the chocolate, and its packaging: "Fondant Dessert" from Nestlé for example is good because it's well wrapped in strong paper. I did the same thing with a cheap chocolate, and the bar flew apart in two pieces when I hit it for the first time...