Peel (optional) and slice apples. Aim for uniform thickness so all of the slices will dry at the same rate. Toss in lemon juice if desired.
4 pounds apples, Lemon juice
In the dehydrator
Place sliced apples on trays, close together but not touching. Dry on 135ºF for 4 to 6 hours or until fruit is leathery.
In the oven
Place wire racks on a rimmed baking sheet. Place sliced apples on racks, close together but not touching.
Set oven to 200 degrees or lower, with the ideal temperature being 135ºF. This will depend upon your oven. The bread proofing setting on newer ovens works well. On older ovens, you'll likely have to set it at the lowest temperature. Even then, you may need to prop the door open to prevent it from getting too hot. (You don't want to cook the apples!)
The timing for this is entirely dependent upon the temperature you use. Dried apples are done when they're leathery.
In the air fryer
Place sliced apples on trays, close together but not touching. Dry on 135ºF for 4 to 6 hours or until slices are leathery.
Notes
Conditioning the dried fruit will help redistribute the moisture evenly. Cool fruit thoroughly and place loosely packed pieces in a large jar. Seal the jar and let stand for a week, shaking it daily to break up the fruit. The excess moisture in some pieces will be reabsorbed by drier pieces. If any condensation appears on the inside of the jar itself, there’s still too much moisture in the fruit. Return it to the dehydrator to remove more moisture.
Adjust the quantity of apples according to how many you have on hand. You'll use the same cooking time no matter how full your dehydrator or air fryer is.
Peeling the apples is optional. The apple peel can tend to be a bit tougher than the flesh when dehydrated, but many people simply skip peeling to save time.
Peeling and coring the apples before you slice them allows you to make full apple rings.
Sliced apples will oxidize and turn brown. To prevent browning, toss the sliced apples in a bit of lemon juice to help prevent browning.