I posted a recipe for elderflower cordial a couple of years ago but I thought I'd give you an updated version rather than send you to the depths of my blog!
So, it's really easy to make, all you need are about ten elderflower heads, 600ml water, 800g caster sugar, one lemon, grated and juiced, and 45g citric acid (buy it from a chemist).
Dissolve the sugar in the boiling water and leave to cool for ten minutes. Add the lemon peel and juice, the citric acid and the elderflower heads (having checked they're free of bugs first!). Give it a good stir and then cover and leave overnight.
Pour the elderflower mixure through a sieve and then fill up your sterilised bottles (to sterilise I put them in the oven at 100 degrees C for ten minutes). This recipe makes about one litre of cordial and it's really quite strong so a little goes a long way. Keep the cordial in the fridge and you should be able to enjoy it for a month.