There are two kinds of seeing: passive seeing and active seeing. You can do many things while passively seeing, but drawing accurately, whether directly or from your visual memory, requires active seeing.
As you go through the day you are in a constant state of visual observance. However, most of that seeing is passive. Unless you frequently look at the same thing over a long period of time you probably have no idea of the precise visual aspects of it or its surroundings. That is because we’re wired to revert to passive seeing until we need to be active. But for that wiring, our brains would become so overloaded that we would cease to function (think ADHD on steroids and caffeine!).
Active seeing is like drawing in your mind. As you observe you consciously think about the various aspects of the image. This is also called analyzing. Acquiring this skill is what cast drawing (especially through Sight-Size) is all about.
Relative to memory drawing, to actively see you would stare at your subject and pretend that you’re actually drawing it. You might even trace the subject in the air with your finger. You would analyze it as you go (i.e., ‘this edge is longer than that one, that edge is at a steeper angle than those edges, this value is lighter than the rest, etc.’).
Nothing Visual is an Island (other than actual islands)
Out of context few things have meaning. That is especially true for making sense of visual input. A short line is only short relative to a longer one. A dark value is only dark relative to a lighter one. I could go on.
My point is that active seeing often requires comparison. When practicing (and even when not) that comparison needs to be between the visual aspects within the scene itself.
Effective memory drawing requires something I call a control. The control is a constant; it stays present within the scene or subject and it is the standard to which you will compare your drawing of the target (the scene or subject you are attempting to memorize).
A part of many memory drawing exercises is a line or group of dots that are superimposed on the subject. They are the control and would be directly copied onto your exercise. They would then be referenced as you memorize and as you draw.
The Visual Impression
Another form of active seeing is perceiving the visual impression. When you consciously stare at a scene, taking in the whole of it while intentionally avoiding comparisons, you are trying to perceive the visual impression.
As you look at your memory drawing subject you will often switch back and forth between trying to see the visual impression and analyzing the subject.
There is one caution to heed before you commence any type of memory drawing. When practicing you should not to use formulas. Formulas are routines like: how many heads tall the figure is, idealized anatomy, simplifying the subject into basic forms (ovoids, cubes, etc.).
Why? Because a formulas supersede your visual memory.
That said, formulas do have their place, but not until performing (drawing from memory to actually create or adjust your artwork). When practicing you are trying to improve your visual memory, not your mental recall of a formula. Using formulas during that practice will stunt your progress.