Remove any insects from the elderflower blossoms. Do not wash them!
Combine sugar and water in a saucepan over medium heat, stirring until the sugar has dissolved. Remove from heat and allow the syrup to cool to room temperature.
Place the elderflowers and lemon slices into a large container (or a pot) and pour the syrup over the lderflowers and lemons. Cover and let steep for 24 hours at room temperature or in the frige.
Strain the syrup through a fine-meshed sieve lined with a cheesecloth or a paper towel and add citric acid. Stir well then pour the syrup into sterilized bottles or jars.
Once opened store the bottle in the fridge.
The citric acid adds flavor and acts as a preservative to make the syrup keep longer. You can find it in any grocery store. You can also omit it and simply add some lemon juice instead.