You've seen this done before in this series.  The bowl shape is simply planished to a finish and the outer edges are tucked in just a little bit.  The proceedure here is to hammer in concentric circles in a series of rows to insure an even distribution of the stresses in the metal.   If you run into trouble and the shape begins to look oblong then simply anneal the piece and then make the correction. When the copper is soft it's very forgiving so there isn't any need to be overly concerned if you don't form the shape right the first time.