Clean the fruits very carefully. It's best to scrub them with a brush in order to get rid of all traces of the chemicals they've been sprayed with.
Cut the citruses into thin slices along with the rinds but get rid of the seeds. Roll them in sugar on both sides and arrange them in a tray in which you've put baking paper.
Put thus prepared slices in a preheated 320°F (160 °C) oven until the sugar is absorbed. Then turn the oven down to 176°F (80 °C) and dry them about 30-40 min., without opening the oven door.
Don't let them dry completely or they'll end up too hard. Store them in a closed jar.