If using a dehydrator you can set it to about 130°f (~55°c) or you can use an oven and set it as low as it goes. 170°f (~75°c) works fine.
Cut citrus into 5mm wheels and place on a wire cooling rack in the oven for 6–10 hours. Time depends on the size of the fruit and the thickness of your wheels.
They’ll go through a sticky phase before they’re completely dry. When they’re fully dehydrated, store in a jar almost indefinitely.