Shading is what makes a drawing pop and prevents it from feeling flat. A big part of the illusion of three-dimensions in good drawings is the shading. But shading is tough to master, especially when you are trying to shade something from your imagination or memory. Shading can imply lines as well. Think about the two small ridges between your nose and upper lip. While you could draw lines for them, that would make them unrealistically pronounced. Try shading them in instead, darkening the areas around them lightly to make them “appear” in the middle of the dark patches. Shadows are created because they are exposed to less light then other parts of the scene. Where the light is coming from, what type of light it is, and even the time of day will all affect your shadows. Shadows are created on opposite sides of the light. For example, if you place a ball down and shine a light on it from the right, the left side of the ball will appear darker. This is where you would shade if you were drawing the ball. The edge of the shadow is how quickly it disappears. Think of trying to make a shadow puppet—when your hand is close to the light and the wall, there is a hard edge where your shadow and the light meet; but when your hand is farther away you shadow softly fades into the light. Note however, the all shadows have a slightly soft edge. The difference between shading and contour drawing is the fading at the edges.  Direct light, like spotlights and clear, sunny days, creates dramatic shadows with hard edges. Indirect light, light far away lights, multiple lights, or cloudy days, create softer, muted shadows with faded edges. Make soft, gentle lines around the edges of your shadows before starting them so that you know where you are going to.  Map out the highlights: where is the light the strongest? Is there any glare? Sketch out the shading: where does the shadow on each object start and stop? Contour any hard shadows. Are there any dark shapes created by the light, like a person’s shadow in the sun? Shading is the...

TL;DR in Indonesian:
Ketahuilah bahwa mengarsir memberi kedalaman pada objek yang digambar. Pikirkan tentang sumber cahaya. Perhatikan tepian arsiran tersebut. Petakan arsiran Anda sebelum memulai. Fokuslah pada transisi bertahap. Baurkan arsiran Anda. Berlatihlah untuk mengarsir objek-objek sederhana. Pelajarilah berbagai jenis shading atau gambar bayangan.