To a pot, add the sugar and water. Bring to a boil and stir until all the sugar has dissolved. Add the citric acid.
Cut the fresh flowers from thick stems (small stems are ok) and keep them in a bowl.
For best results, use organic, unwaxed lemons. Wash the rind under hot water with a little bit of soap. Cut them into thin slices.
Alternate adding citrus slices and elderflowers to a half-gallon mason jar.
Pour the hot sugar water over the elderflowers and lemons. Stir well and put the lid on. Keep the jar in a cool place for three days. Once or twice every day, either stir the mixture or shake the jar.
After three days, using a fine mesh strainer, strain the syrup.
To make this homemade elderflower syrup last longer, in a pot over medium heat bring it back to a boil. Immediately fill the hot syrup into clean jars or bottles.