Preheat oven to 225 degrees. Line 2 baking sheets with nonstick baking mats. Core apples, then slice paper-thin with a mandoline or a very sharp knife.
Arrange slices in a single layer on prepared sheets. Sprinkle evenly with sugar. Bake, turning twice, until dry and edges begin to curl, about 1 hour, 5 minutes. Let cool completely on sheets on wire racks. Dried apple slices can be stored in an airtight container at room temperature up to 1 month.