Stone and wood carving are techniques that involve cutting into the material without being able to add on more when needed so it is much more precise and important to get it right. With stone carving the main technique is to plan out the shape with sketches and detailed drawings and sometimes using lines on the stone, then rough out this shape and finally refine it. A sculptor must assess what material they are working with, thinking about the brittleness and texture of the stone. Professional sculptors now days use pneumatic chisels to get rid of the large bit of material that they don't want, making the work time faster. Its is then a case of chisels and rasps to take away a shape the material. Some sculpting companies make full scale clay versions of their intended to work in order to show clients and they use this to work out measurements, sometimes make a rig that they can transfer across to the stone when needed to check the size and shape.
One website which I found useful, gives you an initial step by step for beginner sculptors. Follow the link below to view.