In his book Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters, Hockney describes the process of using a camera lucida:
Basically, it is a prism on a stick that creates the illusion of an image of whatever is in front of it on a piece of paper below. . . . When you look through the prism from a single point you can see the person or objects in front and the paper below at the same time. . . . You must use it quickly, for once the eye has moved the image is really lost. A skilled artist could make quick notations, marking the key points of the subject’s features. . . . After these notations have been made, the hard work begins of observing from life and translating the marks into a more complete form.