For 10 people
Wash the citrus thoroughly. Finely zest 1–2 oranges and the lemon (avoid the white pith). Juice all the oranges and the lemon. Gather the seeds and membranes in cheesecloth and tie it.
Bring about 700 ml of water to a simmer, turn off the heat, add the flowers and the zest, cover and infuse for 20 min, then chill for 8–12 h.
Strain finely without pressing the flowers. Add the citrus juice to obtain about 1 liter of flavored liquid.
Add 600–700 g of sugar per liter of liquid (600 g less sweet, 700 g more classic) and submerge the seed sachet. Stir.
Heat, stirring, to a rolling boil; skim if needed.
Continue cooking for 12–20 min, until 105 °C.
During cooking, sterilize jars and lids.
Remove the seed sachet, jar while hot up to the rim, close, invert for 2 min, then turn upright.
Let cool and rest for 24–48 h before serving. Store away from light; refrigerate after opening.