Rinse the etrog. Cut it lengthwise and slice very thin. Remove any seeds.
Soak the fruit overnight in water, making sure to fully cover it. Place it in a pot and bring it to a boil.
Remove the etrog and change the water and bring to a boil again. Then remove the fruit and weigh it. Match with an equal amount of sugar (no need for any water).
Cook over low flame for about 45 minutes or until it takes on a jam-like appearance.