Find a location with good light for a portrait shot. Place your subject some distance in front of a simple background and select a wide aperture together with a moderately long focal length such as 100mm on a 35mm full-frame camera (about 65mm on a cropped-frame camera). Take a viewpoint about one and a half metres from your subject, allowing you to compose a headshot comfortably within the frame. Focus on the eyes and take the shot.
I was probably a bit close to my husband for this exercise. However, it does demonstrate the way the shallow depth of field makes the subject stand out. Taken on a cropped sensor 4/3 Lumix GX7 set on Aperture Priority, 1/320 @ F 5.6, FL 42mm.
Given a very helpful comment on my blog regarding the portrait shot of my husband, I wonder if this would be a better option for the exercise. The background may still be a bit cluttered but the lighting is better. Taken with a Canon 5D Mk III using a 24-105mm lens and zoomed at 73 mm. 1/320 @ f4.0.