This is the easiest part, but it will still take some time to learn to do really well. I used to groan when I'd see these - I mean come on - this is simple stuff, right? We all know how to do it. Right? Well, if you really do then you're well on your way already. But most art students actually don't know it nearly as well as they assume they do.
Students think that just because they've seen the basic form shading exercises many times and read about how it's done that they already know how to do it. They're usually wrong. Understanding how to do it is only the first step - You don't actually know it until you can draw it - and draw it well. You should do these exercises every so often to gauge your progress.
Drawing Lesson - A Theory of Light and Shade
Starting to Draw: Light and Shadow
When you've studied these lighting principles it's also extremely helpful to set up little still lifes and draw them, using objects similar to the basic forms, maybe eggs, apples, blocks, whatever you can find. Try to only use one light source and switch off other lights, close window shades, eliminate extra light as much as you can to simplify the lighting.
An important concept you'll encounter when doing these is the core shadow - the core of a shadow, or the darkest part of the shadow, which lies between the lighted plane and a dark plane (I'll be covering planes in depth soon). Also, and a necessary partner to the core shadow - reflected light or bounce light.
One important thing to take careful note of (and always keep this firmly in mind whenever you're drawing or painting - it's one of the vital keys) - reflected light is never as strong as direct light. Note in the drawing above you can clearly see which planes are lit directly and which are receiving only reflected bounce light. You should never put any tone into a shadow area that approaches being as bright as even the darkest tone in the lighted area (the halftones). This cannot be stated strongly enough - it's one of the essential principles used by the masters to simplify drawings and paintings so they're easily readable. In fact this is why the light tones are called halftones - it's a reminder. Divide the value scale into two separate sections - darks and lights - in fact it's best if you leave a gap - for example use values 1-4 in light and 6-9 in dark. No 5 at all in the drawing. This helps to really clarify exactly what is light and what is shadow.
Tintoretto's drawing perfectly illustrates the real importance of learning to light basic forms - when you understand it then you can light more complex forms and make them look solid and real. There are a few other things he's doing here that I'll be discussing in the next followup too, that help to make the drawing clearly understandable. And after a couple more posts you'll really see how all this stuff ties together into an amazing system.
Ok, that's it for #1 rendering basic forms properly in terms of light and shadow - the other 2 entries are about #2, the forms of the figure and how they relate to each other. Be there or be square!