Preheat the oven to 150° (if you have a convection oven, use that setting)

Wash apples well.

In a large bowl, mix citric acid or lemon juice with the water.

It is up to you whether or not to peel your apples. I never do, but we primarily use them for snacking. If you want to preserve your apples for baking, you might decide to peel them.

Core and cut your apples into thin slices, 1/8" thick. I have a coring tool that is handy if you core apples regularly, but a sharp knife will work as well. And I use my mandoline to slice my apples.

Put your slices in the citric acid/lemon water mixture and let them soak for 5 minutes.

Place the sliced apples in a single layer on cooling racks, like a wire rack you use to set cookies on when they are cooling. I have one rack with small gaps that is perfect for dehydrating apples. But my other racks have larger spaces that the apples can fall through. I will layer some of the others, which makes the gaps smaller.

If you want to sprinkle some spices over your apples, now is the time to do that.

Place apple slices in the oven. If you are baking two sheets at once, make sure that your oven racks are several inches apart, though neither of them should be right at the bottom or right at the top. At the end of the day, you want as much airflow as possible.

Stick a folded potholder at the top of the door when you close it so that the oven door doesn't close all the way. An inch of space is fine. This allows any moisture to escape from the oven and increases airflow.

Let your apples dry for 4 hours or 3 hours with a convection setting. At this point, they should be dry and crispy. Take one from the oven and let it cool before deciding if it is as dehydrated as you want. If not, let them go another half hour or hour. It is tough to 'overcook' your dehydrated apples.

Once cooled to room temperature, store your dehydrated apples in an airtight container. You can always 'refresh' them in the dehydrator if they become less crisp after storing.