This section is from the "The elements of drawing & the elements of perspective" book, by J. M. Dent & Sons. Also see Amazon: The Elements Of Drawing & The Elements Of Perspective.
You cannot do too many studies of this kind: every one will give you some new notion about trees: but when you are tired of tree boughs, take any forms whatever which are drawn in flat colour, one upon another; as patterns on any kind of cloth, or flat china (tiles, for instance), executed in two colours only, and practise drawing them of the right shape and size by the eye, and filling them in with shade of the depth required.
In doing this, you will first have to meet the difficulty of representing depth of colour by depth of shade. Thus a pattern of ultramarine blue will have to be represented by a darker tint of grey than a pattern of yellow.
And now it is both time for you to begin to learn the mechanical use of the brush; and necessary for you to do so in order to provide yourself with the gradated scale of colour which you will want. If you can, by any means, get acquainted with any ordinarily skilful water-colour painter, and prevail on him to show you now to lay on tints with a brush, by all means do so; not that you are yet, nor for a long while yet, to begin to colour, but because the brush is often more convenient than the pencil for laying on masses or tints of shade, and the sooner you know how to manage it as an instrument the better. If, however, you have no opportunity of seeing how water-colour is laid on by a workman of any kind, the following directions will help you: -
Get a shilling cake of Prussian blue. Dip the end of it in water so as to take up a drop, and rub it in a white saucer till you cannot rub much more, and the colour gets dark, thick, and oily-looking. Put two teaspoonfuls of water to the colour you have rubbed down, and mix it well up with a camel's-hair brush about three quarters of an inch long.
Then take a piece of smooth, but not glossv, Bristol board or pasteboard; divide it, with your pencil and rule, into squares as large as those of the very largest chess-board: they need not be perfect squares, only as nearly so as you can quickly guess. Rest the pasteboard on something sloping as much as an ordinary desk; then, dipping your brush into the colour you have mixed, and taking up as much of the liquid as it will carry, begin at the top of one of the squares, and lay a pond or runlet of colour along the top edge. Lead this pond of colour gradually downwards, not faster at one place than another, but as if you were adding a row of bricks to a building, all along (only building down instead of up), dipping the brush frequently so as to keep the colour as full in that, and in as great quantity on the paper, as you can, so only that it does not run down anywhere in a little stream. But if it should, never mind; go on quietly with your square till you have covered it all in. When you get to the bottom, the colour will lodge there in a great wave. Have ready a piece of blotting-paper; dry your brush on it, and with the dry brush take up the superfluous colour as you would with a sponge, till it all looks even.
In leading the colour down, you will find your brush continually go over the edge of the square, or leave little gaps within it. Do not endeavour to retouch these, nor take much care about them; the great thing is to get the colour to lie smoothly where it reaches, not in alternate blots and pale patches; try, therefore, to lead it over the square as fast as possible, with such attention to your limit as you are able to give. The use of the Exercise is, indeed, to enable you finally to strike the colour up to the limit with perfect accuracy; but the first thing is to get it even, the power of rightly striking the edge comes only by time and practice: even the greatest artists rarely can do this quite perfectly.
When you have done one square, proceed to do another which does not communicate with it. When you have thus done all the alternate squares, as on a chess-board, turn the pasteboard upside down, begin again with the first, and put another coat over it, and so on over all the others. The use of turning the paper upside down is to neutralise the increase of darkness towards the bottom of the squares, which would otherwise take place from the ponding of the colour.
Be resolved to use blotting-paper, or a piece of rag, instead of your lips, to dry the brush. The habit of doing so, once acquired, will save you from much partial poisoning. Take care, however, always to draw the brush from root to point, otherwise you will spoil it. You may even wipe it as you would a pen when you want it very dry, without doing harm, provided you do not crush it upwards. Get a good brush at first, and cherish it; it will serve you longer and better than many bad ones.
When you have done the squares all over again, do them a third time, always trying to keep your edges as neat as possible. When your colour is exhausted, mix more in the same proportions, two teaspoonfuls to as much as you can grind with a drop; and when you have done the alternate squares three times over, as the paper will be getting very damp, and dry more slowly, begin on the white squares, and bring them up to the same tint in the same way. The amount of jagged dark line which then will mark the limits of the squares will be the exact measure of your unskilfulness.