Slice off the top, stem side, of the apple and then slice the apple into thin rounds, somewhere between 1/8″ to 1/4″ thick. A mandoline slicer works best so they are all the same thickness. Remove the seeds or the whole core. Peel can be left on or taken off.
Toss the sliced apples with cinnamon and sugar (optional) and arrange in a single layer in your dehydrator or onto a parchment paper-lined baking pan.