What is highlighter and what does it do?
If contour is a painted shadow, highlighter is painted light. It sits on the high points of your face, the places that catch the sun first, and makes them look a little more forward, a little more polished.
Used well, it reads as healthy skin. Used badly, it reads as sparkle. The line between the two is placement and particle size, not price.
Where light naturally lands
Tops of the cheekbones. The brow bone, just under the highest part of the arch. The bridge of the nose. The cupid's bow. Sometimes the centre of the chin and forehead, in the smallest amounts.
Notice the pattern: these are the spots that already shine a little by late afternoon. Highlighter does not invent glow, it turns up what is there. That is why a placement that suits your face shape matters more than the product itself.
Choosing a formula
Liquid and cream highlighters sink into the skin and look like skin. They are the safe starting point. Powders sit on top and show more, which is lovely in photos and less lovely in office lighting.
Whatever the formula, look for a fine pearl rather than visible glitter. If you can see individual sparkles on your cheek from arm's length, the particle is too big.
A small warning
Highlighter exaggerates texture. Pores, fine lines, dry patches all get louder under a spotlight. If an area of your skin is having a textured week, skip it. Glow somewhere else.