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.
Placing The Head Rest. When placing the head rest do not adjust it into place until you have obtained the proper position and lighting; then gently slip the rest back of the head-above the neck. To place it higher will tend to crowd the head downward. By placing it back of the head, at the junction of the neck and head, the head will tip in a natural manner. Do not crowd the rest against the head, but place it within about half an inch of the head; then when you are ready for the exposure the subject will unconsciously lean against it. When placing the rest, see that no portion is within range of the lens, as it is quite difficult to erase it in the negative. For a suitable head rest, see Illustration No. 8.