Learning how to fold ingredients into whipped egg whites is an important baking skill to master.
Mixing too much deflates the whites and results in a dense baked good, while folding too little leaves streaks of unmixed batter.
Always pour dry ingredients over wet ingredients. With a large plastic spatula, cut through the middle of the mixture, then rotate your wrist to bring the batter up the side of the bowl. Turn your wrist even more to turn the batter back over itself — in other words, fold! — again toward the center of the bowl.
Repeat this stroke, rotating the bowl and cutting and lifting the batter where it most needs blending. You may not think you're making much headway at first, but resist an impulse to stir; that will deflate the egg whites. Take your time. Gently fold until no streaks remain while keeping the batter fairly stiff.
For macarons, with the goal of the batter being almost pourable, it may seem that you're deflating the batter too much. Fold until the batter slides in a thick ribbon from the spatula, and then not a moment more.