This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Handling The Diffusing Screen. A good plan is to experiment with the diffusing screen whenever it is possible to obtain a subject. Place the sitter in a chair, within about 3 feet of the light, raising and lowering the diffusing screen while watching the variation of light on the subject's face. Locate the subject at various distances from the light and manipulate the diffusing screen first with the curtains entirely closed, then partly separated, etc., noting the different effects. By experimenting at every opportunity you will soon be able to observe the most delicate effects, and within a reasonable time you will be able to manipulate the screen to produce any effect desired.
Placing Reflector At Proper Angle. To overcome this difficulty practice placing the reflecting screen at a variety of angles, also at various distances from the subject, carefully noting the effects produced on the shadows. Always bear in mind, that the back of the head and the ear on the shadow side should be but slightly illuminated. Aim to reflect the light between the nose and the cheek bone on the shadow side.
273. Refer to the illustration of floor plan and observe the angle at which the reflecting screen should be placed. Frequently strong high-lights are subdued by reflecting light into the shadows, thus softening the contrast between high-lights and shadows. It is important that the reflector be placed at the proper angle, because incorrectly located it will be responsible for flatness instead of the desired roundness in the portrait. While the screen is placed on the shadow side, it must be located slightly in front of the subject to reflect light upon the fore-part of the shadow side of the face, permitting the light on the rear part of the head to gradually blend off into shadow. With practice you will soon learn to overcome many obstacles with the reflecting screen.
Diffusing High-Lights. When the high-lights are strong, the shadows are dense (black), and the result is a lighting full of contrast, to overcome which the light should be subdued. This must be accomplished through the manipulation of the diffusing screen. It may be found necessary to use a double set of curtains on the diffusing screen; sometimes the pinning of a single sheet of newspaper over it will secure the desired result. Beware of over diffusion, as it produces flatness.
Illuminating Background. To properly illuminate the background, the end furthest from the window, on the shadow side of the subject, should be turned toward the window at an angle that will cause the light to spread evenly over the entire ground. By this method the end of the ground furthest from the light will be evenly illuminated, and can be reproduced as it is painted. Experiment by placing the ground at different angles, carefully watching the varying degrees of illumination.