Cut the oranges, lemons, limes, grapefruit, and apples into thin slices, pat them with paper towels to absorb excess moisture, and lay them out in an even layer on parchment-lined baking sheets. Bake at 225˚F for 1 1/2 to 2 1/2 hours. Flip the fruit slices occasionally. Keep an eye on the fruit to make sure it doesn't start to burn. Smaller, thinner pieces of fruit will dry faster. It is ready when the fruit is dry and no longer sticky.