I drew both the sphere and this yarn ball from life, however, you can accomplish the same thing by drawing from my scans.
A “Soft” pencil (B, 2B, or 3B.)
Drawing Paper Marked “smooth” or “medium.”
First Draw The Shaded Sphere
Add Yarn Line Detail Over the Ball
Holding the pencil in a writing grip, sketch the pattern of the yarn strands over the top of the smooth ball.
Using the kneaded eraser, gently tap and stroke out the highlights along the strands of yarn.
What to look for:
• Highlights along strands that stick out farther than the surface of the basic ball shape catch the light and are more highlighted.
• The way highlights run along the center of a yarn strand while the edges stay in shadow, because the strands are thick and rounded.
Add Shadow Details
Darken areas to “push” them back into the surface of the original sphere.
The shadows darken along the edges of some of the strands more than others, depending on how they angle away or are shaded from the light.
The larger body shadows vary in darkness also depending on how the light now strikes them.
Some of the shadows have soft (indistinct or blurred) edge lines and some of them have hard (cleanly drawn) edges. Again, this has to do with how the light affects them.
Finally, change the edge shape of the ball and the cast shadow to show how the yarn ball’s surface undulates.