Practice using calipers to measure. Place upper "jaw" of calipers on one point of the face, and slide the lower jaw down until it is positioned on the second reference point. The ruler on which the lower "jaw" of the caliper slides will indicate the distance between the two points.
Measure the general dimensions of the head first. Measure the distance between the top of the head to the chin, left cheek bone to right and the tip of nose to the back of the skull.
Measure basic placement of features. Measure hairline to chin, hairline to nose tip, nose tip to chin, nose tip to ear, ear to chin and ear to root of nose. These measurements set up the accurate rough placement of features.
Measure distance between features. Measure distance between eyes, eyes to hairline, between eyebrows, eyes to lips, base of nose to center of lips, lips to chin and point of chin to edge of jaw.
Measure size of features. Measure thickness of eyebrows, width and thickness of nostrils, width of philtrum (vertical groove above upper lip), thickness and width of lips, width of eyes, width of nose root, width of base of nose and width of nose tip. It is these small details that make sculpture that will wow with its accuracy.
Use these recorded measurements to build your sculpture, or to double-check your work. The more measurements you take and the more detailed your notes, the more realistic and accurate your sculpture will be.