This is the opposite of the previous lesson. A low key photo is pretty much black on black, or at least very dark on dark. This kind of photo can create a sense of intimacy, foreboding, sadness, and / or heaviness. You might have a face or object rimmed with light in a silhouette, but dark on the side towards the camera and dark background.
The problem with shooting dark on dark is that the camera will try to lighten the image up making the picture look washed out and grey.
For this shot you'll need a really dark cloth, preferably black, and an object that is dark or has some dark tones in it. You could shoot a portrait of a dark haired person in dark clothing against a black or dark background for a low-key portrait.
Note: Shooting an object or person that is very light or white against black has a different effect and is not really considered "low key", although it can be striking image anyways.
Exercise: First, shoot the image with what you camera says is the right exposure. Very few in-camera meters will render this scene accurately. Now, take a meter reading on something dark/black that has the light hitting it and close the aperture two stops (i.e. if it is ƒ1.8 you'll want to go to ƒ4.)
Compare the two images and see what difference it made.