What are the skills of sketching?

Sketching is a basic building block of an artist's talent. Like any other thing you do, art takes practice, and you practice by sketching.
Sketch anything and everything your see. Keep a sketchbook and drawing materials with you wherever you go, and don't be afraid to take them out and use them.
Your sketches help you to develop your ability to capture your subject. They help you to develop your perspective and proportion and they become a source of future work and are also an Excellent record of how and how much your talent has changed, grown and progressed.