Briefly rinse the elderflowers with cold water. Cut the lemons into slices.
Heat the sugar and citric acid with 2 liters of water until the sugar is completely dissolved. Allow to cool a little.
Place elderflowers and lemon slices with the mint in a tall glass. Pour the syrup on top. Cover the jar with a cloth (or napkin) and place in a sunny place for 2 days.
Take out the lemon slices, mint leaves and flowers. Bottle the finished syrup through a coffee filter.