Take a style of silver or brass, or anything else provided the point is
silver, sufficiently fine (sharp) and polished and good. Then to acquire
command of hand in using the style, begin to draw with it from a copy as
freely as you can, and so lightly that you can scarcely see what you
have begun to do, deepening your strokes little by little, and going
over them repeatedly to make the shadows. Where you would make it
darkest go over it many times; and, on the contrary, make but few
touches on the lights. And you must be guided by the light of the sun,
and the light of your eye, and your hand; and without these three things
you can do nothing properly. Contrive always when you draw that the
light is softened, and that the sun strikes on your left hand; and in
this manner you should begin to practise drawing only a short time every
day, that you may not become vexed or weary.
_Cennino Cennini._