I find the swing-top bottles are the best for storing these. The citric acid will help it to store longer, but I have never bothered with it.
Place the flower heads in a large bowl together with the orange and lemon zest. Bring 1.5 litres water to the boil and pour over the elderflowers and citrus zest. Cover and leave overnight to infuse. Strain the liquid through a piece of muslin and pour into a saucepan. Add the sugar, the lemon and orange juice and the citric acid (if using). Heat gently to dissolve the sugar, then bring to a simmer and cook for a couple of minutes. Use a funnel to pour the hot syrup into sterilised bottles. Seal the bottles with swing-top lids, sterilised screw-tops or corks.
Dissolve sugar in water, and leave to get cold; add lemons, flowers and vinegar. Cover loosely with a tea towel and leave for 24 hours. Strain and bottle, try after a fortnight.