As you move the light from side to side, the shadow of their nose will move around.You want the edge of that shadow to fall right along their smile crease.
Scouting your locations in advance, and determining the time of day you’ll be shooting, will help you understand and adapt to otherwise unpredictable lighting changes.
If you're using a stand (as opposed to lights hanging from a studio ceiling), begin by adjusting the light so that it's as tall as it can get while still reflecting in your subject's eyes. Now look for the crease in your subject's face that runs from their nose to the outside of their mouth.
If you have trouble seeing it, ask them to smile to get it to pop out a little bit.
The lighting scheme for a single subject is called three-point lighting.
It includes a key light, a fill light and a back light.