To create a low-key image, all you need is your camera, one light source and a dark place with dark colour background. Make use of a fill light or reflector. At times, the light from a computer monitor, TV screen or LED torch on the subject in a darkened room can produce splendid low-key pictures.
As far as camera setting is concerned, use ISO 100 or as low as camera permits and adjust the aperture progressively high to low until there is no ambient light in the frame. Thus, the main precaution to be taken into account is no light should reach the dark or black background.
You may block off all light in and use a slit in a window as the light source to illuminate the subject. Try to keep the light from hitting the camera or the background while taking the photograph.