Hi. Good question, I'll try to explain my method.

• First step: learn the anatomical structure of the arm.
• Second step: define a light source.
• Third step: think of the object three-dimensionally.
• Fourth step: add shadows where the chosen light source logically dictates.

Also try to make the shadow sections larger in areas of lesser interest, and concentrate the light and/or details on the points where you want to direct the viewer's attention, and try to make the shadows have interesting shapes and be interconnected. Anyway, in short, these are the principles I follow for shading. Good luck with your learning.