Block out the area that will be silhouetted so that you know the space you will be filming in and how much you need to light.
Light the background with flat lighting as opposed to traditional three-point lighting. Three-point lighting uses a key light, fill light and back light. To create a flat lighting look, avoid using the fill light and back light on the background. Instead use one, or even two, key lights to create a flat light background.
Frame the subject in front of the background. The better the outline of the subject, the more dramatic the silhouette effect will look.
Light the subject from the back. Position the back light directly behind the subject for a complete blackout silhouette effect. Adjust the angle and height of the back light to your desired effect for more or less visibility.