Because the goal is to add warmth to your base makeup and fake a healthy glow, bronzer should be applied on top of your foundation—doing it the other way around (that is, bronzer before foundation) can leave your skin looking patchy.
Pro Tip: For a glowy, beachbound look, mix a liquid illuminator, like Born To Glow Liquid Illuminator, into your foundation before applying your base and bronzer, then add blush to your cheeks. Learn where to put blush in our article How To Apply Blush and Flatter Your Face Shape.