Place the fill light next to the camera as close as possible without creating any shadows. The job of the fill light is to give detail to the shadows that are produced by the main light. You can make the shadows lighter or darker depending on the key of the photograph. (If the subject were wearing a dark outfit against a dark background, it would be proper to allow deeper shadows. If the subject has a white outfit on a white background and you allow a dark shadow, the shadow would be the main focus instead of the subject.)
When it comes to ratios between the main and the fill lights, the photographer needs to control the amount of fill light based on the key of the photograph. Most of all, make sure the shadows have detail—there’s no such thing as a black shadow in a portrait. The fill light is normally a soft light or large light size.