Take small firm naartjes, then grate or peel very thinly. Make four incisions in the sides, and place in fresh water for three days and nights, changing the water twice daily. Boil in fresh water until quite tender, then drain. Make a syrup of two parts of water to one of syrup, allowing the same amount of sugar as fruit; add the fruit to the boiling syrup and cook for several hours until clear and transparent. Put into dry sterile jars, adjust the rubbers and lids, and seal tightly.
Another way is to cook the naartjes the first day for one or two hours in the syrup, then to put aside until the next day and cook again for an hour or so, the same process to be repeated the third day.