Answer:
Which “modeling clay” are you thinking of? (There are 4–5 basic types of “clay” these days and they can all be modeled/shaped, but only one is “true modeling clay”—i.e., plasticine-type clay, the type that never hardens.)
How you’d change the shape of a clay would depend on how stiff it is, how smooth it is and how smooth you’d want the final surface to be, what you wanted to make/achieve, whether you needed fine shapes/detail or just grosser shapes/detail, and whether the clay was still unhardened or had already been hardened.
Most often, you’d shape unhardened clays with your hands and fingers, but also sometimes with various tools.
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