Some important notes taken from the skills workshop and the 3-point lighting handout:
The key light is the main source of light just as the Sun is. […] If there is already a light source present make sure you position the light in the same direction.
Whatever side you decide is best for the key light to be positioned it must be placed in the direction of the subject at an angle between 30 and 45 degrees from the side of the camera and raise the light so it is at a 45 degree angle above the subjects face. If your subject is wearing glasses you should raise the light higher than 45 degrees to avoid glare on the lenses.
The fill light serves the purpose of eliminating any shadows cast by the key light and to give the audience a sense of the ambiance in the room.
If you subject has lighter hair or is bald the backlight needs to be softer.
Once you have set up your lights you will need to look through your camera lens or use a monitor to ensure that everything looks as it should. Ensure that the subject has a nice glint in their eyes but also that the hair is not too bright and appearing over exposed.
A visual example: