METHODS OF PORTRAITURE
There are as many ways to paint a portrait as there are paintings, however, here are two of the more discernible methods.
General to the particular. Start with an image of an average head, a general schema, compare the model to this schema, note the differences and adjust the schematic painting/drawing accordingly.
Measuring using mainly plumbing and horizontals. Eyeball something sight size and use this as a measure for the rest of the image. Sickert, Coldstream and Uglo used this method.
We think that we know what things look like based on past experiences, and this gets in the way of accurate observation. When drawing by observation for the observation to be effective, you need to know what to look for, to discriminate, and decide what is essential, otherwise you will be confused by the sheer amount of information that a head contains. Observe with understanding and always challenge your first perceptions.