Most important, keep you subject between you and source of light. Choose good subject, something or someone that is clear and interesting.
Turn of your flash. you don't need it because you want to keep your subject in shadow. So, you want to meter and correctly expose for the background, which will place your subject in shadow. Digital cameras make it easier to
shoot silhouettes because you can see on the LCD exactly how the image looks, and adjust your exposure accordingly. Focus manually or focus on the edge of the subject where the contrast between subject and background is greatest. Use a small aperture when possible for greater depth of field. When photographing a silhouette of a person, make sure your subject wear tight clothes instead of loose ones and shoot them in profile. The profile of a person whether a full-length or close-up shot of their face, will be more recognizable than if they were facing the camera.