Preheat the oven to 200 °F. Line 2 large baking sheets with parchment paper or silicone mats. I use the unbleached parchment and have not had a problem with sticking.

If desired you can core your apples but I found this unnecessary because when you slice them the seeds easily pop out. Using a knife or a mandolin thinly and uniformly as possible slice apples into round apple chips. Slicing them evenly and thinly helps them all to crisp and dry well and at the same time. Place in prepared pan without overlapping apples. If desired sprinkle cinnamon or cinnamon/sugar.

Place in preheated oven and bake for about 45 minutes, flip over apples on the tray and return to oven to bake for another 1 1/2 to 1 3/4 hours (for a total of around 2 1/2 hours)--I will usually flip them a second time (if I remember) after about another 45 min into cooking time, to make sure they don't stick. At the end of the cooking time, the apples edges will curl and feel dry but may not be completely crisp yet.

Leave the apples in the oven with the door closed for at least another hour or so. This will allow them to fully crisp up. You can even bake them before bedtime and leave them in oven overnight to crisp up.

Allow to cool completely and store in an airtight container.