I assume you mean a 16x16 baseplate, correct? Well, the simplest way would just be to stack bricks and try to create as symetrical a shape as possible. Remember to stack the bricks in such a fashion where, for example, the first 4 layers move inward by one stud, the next two by two studs, and so on. This creates a rounded shape. If you moved inward by the same amount each layer, you would simply end up with a pyramid. See what I'm saying? I could whip up a visual if you really want it.