I used it on my brow bone, bridge of my nose and the sides of my nose and slightly above my cheekbones. When I go out at night, I also use it on my undereye area to highlight it.
And I use it over my foundation, because it's so sheer, if you put it under, it won't make any difference. Don't overuse it though, excess product will sink into my undereye fine lines and make them far more evident.