The proportion of drawings comes from the basic transistion of basic mathematics calculation, but it doesn't mean that you will brace your self in pounding numbers, but of course it is extremely crucial to learn mathematics. In this case, we will use the descriptive ideas to illustrate the importance of cube. I am sure you understand the cube, an object like a box and space inside. This is use to guide the artists and even the novice users to attack the proportion of drawing. For example creating a building, house,landscape and even the human anatomy. Yes, this is part of the so called technical drawing, thru technical drawing anybody can measure the depth and space of the object.
How to start with the cube? draw a box and create a simple version of a 2d drawing, don't push your drawing to hard to reach the end of perfection, just draw it with enjoyment and once your hands got the rythm then create more cubes. Never mind the unlined points just draw until you perfect it, anyway you are not dealing on numbers, but to the angle of proportions.
The cube can highten your illustrations, like for example if you are creating a background and different objects.Drawing in perspective requires a great proportion and understanding the lines and its direction to connect on the optic perception of the viewer.
Skills can be acquired later if you practice your ability(Plato's principle was, to learn more is to practice and gain experience every day), erasures are normal, but once again I would like to reiterate this, don't pressure your self, focus and keep on practicing, there are basic books on drawing, but thru reading and peeling information on the web can increase your knowledge.
