Gently wash the elderflowers then place in the pan.
Using a peeler, remove the zest from the oranges and lemons and place in the pan along with the juice of one orange and one lemon.
Finally slice the remaining oranges and lemons and place in the pan, giving everything a good stir.
Leave covered for 24-48 hours before straining through a clean muslin cloth or tea towel (lining a colander over a big bowl and gently pouring the liquid in is the easiest way to do this).
Using a jug and funnel, pour the liquid into sterilised bottles (running them through the dishwasher should be enough but to be extra sure, leave them to dry in a low oven).
Keep in the fridge until you want to drink it.
Use within about 6 weeks, or freeze some and defrost it when you want it (you could do this in ice cube trays, in which case you would have a ready-made ice cold drink when popped into water!)