This is a photo of the subject for my drawing with the terms used to indicate the tonal steps (modeling factors) labeled.
To start, I do a careful contour drawing plus indicating the location of the high lights, terminator, and cast shadow.
Next, to save the high lights I make everything else several shades darker. For this light halftone I use a #H pencil.
Now I have made a darker mid tone to seperate the light side from the shadow side using a #HB pencil. Still only two values plus the white of the paper.
Finally, I add the darker values indicating the pattern on the light side with a #H or HB, being careful to keep them no darker than the light tones of the shadow side. Then likewise I add the darker values, using a #HB and 2B pencil, indicating the pattern on the shadow side along with the core, occlusion, and cast shadow.