Bring 1 cup of water to a boil. Add the orange slices and cook for 2 minutes. Remove with a slotted spoon and place in the ice bath. Do this for all the citrus you plan to cook.
If needed, add more water, then toss it. Begin the syrup with 2 cups water and 2 cups sugar. This process is called blanching and it makes the citrus peels less bitter.
Add sugar to the water and stir until dissolved. Bring to a simmer.
Add citrus slices in a single layer. Don't stir, but you can use tongs to flip them if they begin to cup — this helps them lie flat.
Cook for 30-40 minutes, until the fruit part of the slices are thinned.
Place a wire baking rack on a baking sheet. Remove the slices from the syrup and place on the wire rack. Let cool completely.
If desired, you can coat the slices in sanding sugar or granulated sugar.
Notes
Use a variety of types of oranges to get different colors. I used navel oranges, cara cara oranges and blood oranges. You can also make candied limes, lemons or grapefruit. The candied slices can be used to decorate drinks and desserts, or you can eat them plain.