Add the flowers in a big jar, or if it is a larger amount, in a big plastic bowl with a lid (you can use a pot too).
Wash the lemons and cut them on thin slices. You can also blend them in a blender to get a stronger lemon taste.
Add citric acid and lemon balm leaves cut in a few pieces (optional).
Pour with cold water and close. Leave it for 24 hours, with occasional stirring, in a dark and cool place.
After that period, drain it with a cloth. Bring the liquid in the pot or jar and add the sugar.
Close and leave for another 24 hours with occasional stirring to melt the sugar.
After that period, the cordial is ready. Collect it in bottles or freeze it in cubes. (You can store in glass bottles if you do not freeze it, but keep it in a dark, cool palace. If you want to freeze, store it in plastic bottles.)