the overall shape
the center line (that runs between the eyes and down the nose)
ear placement (between the eye line and the bottom of the nose)
and the lines for the mouth, nose and eyes.
Paying special attention to angle of the head - the way the eye line tips - up, down or straight on.
You can practice in a coffee shop, train station, on a bus, etc or grab a magazine and sketch as many heads as you can fit on a page.
Leave out the details for this assignment - right now you are looking only for the basic structure. And getting comfortable with the placement and angles.
"You can't do sketches enough. Sketch everything and keep your curiosity fresh.” John Singer Sargent
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