Go out into your garden, or into the road, and pick up the first round or oval stone you can find, not very white, nor very dark; and the smoother it is the better
only it must not shine. Draw your table near the window, and put the stone, which I will suppose is about the size of a in Fig. 5 (it had better not be much larger), on a piece of not very white paper, on the table in front of you. Sit so that the light may come from your left, else the shadow of the pencil point interferes with your sight of your work. You must not let the sun fall on the stone, but only ordinary light: therefore choose a window which the sun does not come in at.1 If you can shut the shutters of the other windows in the room it will be all the better; but this is not of much consequence.
Now if you can draw that stone, you can draw anything; I mean, anything that is drawable. Many things (sea foam, for instance) cannot be drawn at all, only the idea of them more or less suggested; but if you can draw the stone rightly, everything within reach of art is also within yours.
John Ruskin, Elements of Drawing, Exercise VIII, 1857