Modelling form is two skills stacked. First you need a value scale — a reliable run of greys from white to black that you can actually hit on demand. Then you need to know where each of those greys goes once a light hits a rounded thing.
First, Build a Value Scale
Rule a strip and divide it into about five boxes. Leave the first as the white of the paper, push the last to your darkest black, then fill the middle three with even steps in between — a light grey, a mid grey, a dark grey. The test is simple: the jump between any two neighbours should feel equal, with no sudden cliff and no two boxes that look the same. Most beginners cram everything into the middle and never commit to a true white or a true black, which is exactly why their drawings look weak. A full scale, anchored at both ends, is the whole secret to convincing form.
The Five Steps of Light on a Sphere
Shine one light on a ball and the tones always land in the same order. Nearest the light is the highlight — the small, brightest spark where the surface faces the lamp dead-on. Around it is the light — the broad lit area, a touch darker. As the surface curves away you reach the core shadow — and here's the surprise: the darkest part of the ball isn't the very back edge, it's this band just past the halfway turn. Past it the back edge actually lifts a little into reflected light — light bouncing up off the table. And on the surface the ball sits on lies the cast shadow — the dark shape the ball throws, anchoring it to the ground.
Why the Order Never Changes
Once you trust the sequence — highlight, light, core shadow, reflected light, cast shadow — you stop inventing shading and start reporting it. The same five steps wrap a cheek, a nose, a forehead, an apple, a coffee mug. A face is just a cluster of rounded forms, each one obeying this exact run of light. In the toy, move the light overhead, off to the side, low and raking — and watch the highlight, the core shadow and the cast shadow all swing round together, always in the same order, never out of step.