I've made Elderflower fizz and cordial for the past few years and use tried and tested country recipes I was given on a chicken and smallholders forum. They are good, there is no faffing about with yeast and demijohns or anything remotely terrifying like that. Just flower heads and a few basic ingredients, and Bob's your uncle! The good thing about Elderflowers is the season is fairly long for a wild flower, usually four or five weeks. This gives you plenty of time to pick enough to make a decent amount of drink. As all the magic is in the pollen with elder, it acts as a yeast and causes the fizzing, don't pick flowers just after it has rained. Best to try and pick the flower heads in the morning too.
Elderflower fizz recipe
4.5 litres water,
2 tbsp white wine vinegar,
finely pared rind and strained juice of a lemon,
4 large (or 8 to 10 smaller) unwashed Elderflower heads with flowers snipped from stems.
Pour the sugar into a large bowl or pan and add the water. Stir until sugar has dissolved. Stir in vinegar, lemon rind, juice and elderflowers. Cover and leave to stand for 48 hours. I use these large 5 litre buckets with lids,
Strain the mixture through scalded muslin or a jelly bag and pour into clean screw top PLASTIC bottles (at this point the liquid is not fizzy) and leave to stand for three weeks, by which time it will have become wonderfully fizzy, Chill before serving.
Right, a few things I have learned over the years; don't snip each individual flower off it's stem, just cut them all off roughly in one go. You can use glass bottles if you use ones which have held a fizzy drink, as these will have been made to withstand the gasses, but be careful. You must unscrew the bottle tops each day to relieve the fizz, otherwise you end up with a fountain of froth and sticky walls (don't ask me how I know!). Use plastic bottles that have held fizzy pop, the lids are better. Don't pick Elderflowers if they smell like cat pee, the drink will taste like it!
And finally, the finished product does contain alcohol (typically 2%) so be careful who you offer it to and make sure no-one drinks too much and drives!