Set your oven to 200°F (90°C) before you do anything else. This low heat dries the apples instead of cooking them. Let the oven fully warm up while you prep.
Cover two large baking sheets with parchment paper or a silicone baking mat. This keeps the slices from sticking. It also makes cleanup much easier later.
Wash the apples well and pat them dry. Use a mandoline to cut thin slices, about 1/8 inch thick. Pick out any seeds as you go.
Lay the slices on the sheets in a single layer. Keep them close together, but don't stack them. A flat layer helps every slice dry at the same speed.
Dust the cinnamon over the slices. Then flip them and dust the other side too. Both sides should get a light, even coat.
Slide the trays into the oven and bake for 1 hour. The slices will shrink and curl a little. That is a good sign, so don't worry.
After the first hour, turn each slice over. Bake for another hour. This second round dries the other side and builds that crisp bite.
Turn the oven off once the time is up. Leave the apples inside for about 30 minutes. They keep drying and crisping as the heat slowly fades.
Take out a slice and let it cool for a moment. If it isn't crunchy yet, bake in 15-minute rounds. Keep checking until the chips snap nicely.
Lift the chips off the sheet and enjoy them right away. If you want to save some, move them to an airtight container. Make sure they are fully cooled first.