Pour water in a pot, add sugars and citric acid, and bring to a boil. Mix well and let all the sugar dissolve. Remove from heat and let cool down.
Add mint leaves, slices of one lemon and the elderberry flowers - remove as much stem as you can and only use the florets. Mix in, then cover the pot with a cheese cloth or a thin kitchen towel, and let infuse for about a day.
Strain the syrup and pour it in a bottle. Will keep refrigerated for about a month.