Use directional lines to follow the form of the object. For example, use curved strokes when shading a sphere to reinforce its roundness.
Squint at your drawing. If it looks grey and flat, your tones are too similar. Darken your deepest shadows to make the sketch "pop." Combining Sketch and Tone for Pencil Drawing Pr...
Start with basic geometric forms (circles, blocks) to establish proportions. Use directional lines to follow the form of the object