The aim of carving as part of creating a bonsai, is to give character to what might otherwise be rather bland material. It is not a course of action that is obligatory; the nature of the material being created into a bonsai will determine whether or not carving is appropriate, let alone necessary.
If you are using material collected from the wild, take your time and examine it carefully. Envisage the style you would wish that material to become; what is the quality of the bark on the trunk? Where you to have a pine with beautiful plated bark, would you want to lose that bark for the sake of adding another (though not necessarily better) characteristic through carving?
The extent the degree and quality of carving is carried out is largely dependent on the ability of the person carving and cannot readily be taught other than watching expert bonsai artists.
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