01 - In a large pot, bring the water to a boil, then remove from heat.
02 - Add the elderflower heads, lemon zest, and lemon juice to the pot. Stir to combine.
03 - Cover and let steep for 24 hours at room temperature.
04 - After steeping, strain the mixture through a fine mesh sieve or cheesecloth into another pot, discarding the solids.
05 - Add the granulated sugar and citric acid (if using) to the strained liquid and stir over low heat until the sugar has completely dissolved.
06 - Bring the syrup to a gentle boil, then reduce heat and simmer for 10 minutes.
07 - Pour the hot syrup into sterilized bottles while still hot. Seal and let cool.