In this tutorial I'll show you how to make a Mad Hatter's Hat Cake Topper. It can be used as a decoration or a top tier. If you want the hat to have cake inside, simply substitute the polystyrene with cake.
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Place the dummy on a chopping board and use a serrated knife to trim the sides. Cut from the edge of the marked circle down towards the bottom edge of the dummy. Before cutting take a look at the shape of the hat so that you know what shape you want to get. I suggest leaving the marked line (don't cut it off) - it will be easier for you to keep the round shape.