In general, cabinets can be shaped using one of two methods. The first and most common is clicking Edit Top Shape in the Shape Tab of the Product editor. It is important to note that you must have the overall dimensions (depth and width based on the wall the cabinet is placed on) set before making the modifications as the shape will change the proportional dimensions to match the shape you specify, but you must change the size independently.
Alternatively, you can right click on the cabinet and click Edit Shape on Floor Plan. This feature is useful when matching the cabinets to adjacent cabinet or unique wall angles. It is important to note with this method, if you make significant changes, it is best to double check the side assignments to verify the sides were not reassigned to something different.