Yes, we always did it with my grandmother in summer!
Look here, this is for the syrup:
You take around 25-30 elder cluster-heads when the flowers are smelling nicely, but not throwing the yellow pollen all around yet.
You take a big bottle, preferably that type they use for putting away cucumbers... it should be around 5 litres. Okay, it can be a pot, too. It is better if it is a transparent bottle cause you can actually leave it in the window, on the sun - it tastes better!
Fill it up with pre-boiled lukewarm water, put the flowers into it.
Add lemon salt - 25 dkgs (quarter of a kilo)!
Leave it stay, preferably in a warm place for 24 hours.
When done, take the flowers out, filter and add about 4 kgs of sugar - (taste it and decide if you want more or less than that).
Pour into smaller bottles and... there you have it!
There are hundreds of recipee versions by the way. According to some, you can even leave it for 5 days. Just take care to filter it before it gets rotten or decomposed. Lemon does not conserve elder till the end of time... you need the sugar, too!
Enjoy.