Two years ago, a friend bought me a dehydrator for Christmas. I then discovered that dehydrated strawberries are AMAZING. So, after some research I decided to try dehydrating apples in the late summer. I did some searching and then some experiments and after trying a bunch of things, I have figured out how I like them best.
First, you want hard, sweet apples (like Gala or a Delicious variety). Stay away from softer apples (like Cortlands), they don't hold up well to slicing and dipping. Second, you want to create a very simple Lemon Simple Syrup. Into a medium sized sauce pan combine 4-5 cups of water, 1-2 tablespoons of lemon juice, and 1/4 c sugar. Heat and stir until everything is combined and then take off heat.
Once you have the Lemon Simple Syrup created, you can then worked on the apples (this syrup could be used any time you needed to coat apples to keep them from browning). My dehydrator will take between three and four apples. I wash them (I use a vinegar wash), quarter them, and core them. I keep the skins on, but it's totally up to your personal preference.
Slice them 2mm thick. I use my fancy OXO mandolin, and I really suggest using one for uniform thickness. Or, just use a sharp knife and watch your fingers.
After every couple of quarters, I drop them into the syrup in the sauce pan and make sure each slice is covered in water. I keep the slices in the pan until I'm done slicing. DO NOT SKIP THIS NEXT STEP: after all slices are in the sauce pan, drain them. This is super important. They will get soggy and fall apart if not drained. After you drain them, arrange them on the racks per your dehydrator's instructions. Make sure they don't overlap each other so that everyone can dry on their own.