This floral, sweet-smelling syrup is a perfect match with seltzer water or, better yet, champagne. I add lemon to my elderflower syrup to give it some zing, but the aroma is all elderflower. It is a delicate scent, very sweet and just a bit spicy. You need to infuse the flowers for at least 2 days, preferably three; I have seen other recipes that call for four days, but you might get spontaneous fermentation holding it that long.
Snip off the flowers from the stalks into a large bowl or jar that will hold everything. Try to remove as much of the stems as you can; they are toxic. A few stray bits of stems will not hurt you, but you want to minimize it.
Zest the lemons and add it to the bowl, then the citric acid and lemon juice.
Bring the sugar and water to a boil, stirring occasionally to dissolve. Let the syrup cool enough so that you can stick your finger in it without getting burned; you can leave it to cool to room temperature, too. Pour the syrup over the flowers, lemons et al and stir to combine. Cover the bowl or jar and leave it for 2 or 3 days.
When you are ready, strain it through a fine-meshed sieve lined with cheesecloth or a paper towel into a clean Mason jar. Seal the jar and store in the fridge.
To serve, pour 1 to 3 tablespoons of the syrup into a pint glass and add water or seltzer. Or you can add a tablespoon to a glass of sparkling wine, or to a couple shots of vodka or gin.