Place a wire rack on the top of the baking sheet. Add sweet potatoes on the wire rack and bake at 400F. Next, place the baking sheet on top of the rack in the oven. That way, the fries will cook from top and bottom. The air will circulate evenly when you bake the fries on the wire rack, and they will be cooked nicely. And the bottom of the fries will not get burnt.