In making studies, don't try for surface finish; get the facts, and leave all other qualities for the picture. Don't glaze and scumble, but work as directly as you can. Study the structure and texture of whatever you are doing. Understand it thoroughly as you go on, and search out whatever is not clear to you. This is no place for effects; nor for slighting or shirking. If you do not do work of this kind thoroughly, you might as well not do it at all—better; for you are at least not training yourself to be careless.
There are places where you may be careless, but the making of a study is not that place.
Take plenty of trouble with preliminaries. Get all your foundation work true. Have a good drawing, get the groundwork well laid in, and then build your superstructure of careful study.
Don't be afraid of over-exactness, nor of hardness and edginess here. All that is only an excess of precision, and it is just as well to have it. You can leave it out if you want to in your picture, but a groundwork of exactness is not to be despised.
Be exact also with your values. If your study is not sure of its values, it will weaken the results you should get from it later.
Make your studies in the same light as that which the picture will represent. You can paint a picture under any light you please if your studies give you the facts as to light and shade that the truth to nature requires; but studies made in one light for a picture representing another are useless to that picture.
Study of a Blooming-Mill. D. Burleigh Parkhurst.