Be sure to pick Melissa leaves before the plant blooms, that's when they're most aromatic.
Wash the Melissa bunches and put them in a suitable container, sprinkle with the citric acid and pour in the water. Leave to sit as is in the fridge for at least 48 hours.
Then strain through a gauze, add the sugar and stir until completely dissolved.
Optionally, add a little yellow dye for a more appetizing appearance (don't use green to avoid confusion with mint).
Distribute into jars and store in the fridge.
Another option is to sterilize 5 min. in the jars - that way you'll have canned syrup year-round.
It makes for a very refreshing summer beverage when diluted with sparking water or water.