In many cases, contouring is used to highlight the center of the face while downplaying the areas of the face that are out of proportion.
No matter your face shape, the areas to highlight will always be the center of the face. Apply the highlight shades at the center of the forehead and the center of the chin to pull the eyes away from the sharper edges of the face.
In the case of square and rectangular faces, we recommend contouring the forehead at an angle and adding the darker color along the jaw and up to the ears. A rectangular face will also need some contouring at the hairline.
A heart-shaped face and a triangular face has similar concerns: The face is unbalanced at the forehead and chin. In both cases, you want to contour the area below the cheekbones, and highlight the area below the eyes. A heart-shaped face will also want to contour the temples.
In the case of a diamond-shaped face, you want to contour the area below the cheekbones to the top of the jaw. It creates a softer appearance and helps reduce the impact of the angular lines of the face.