Pick the flower heads in the morning. When you get home, cut the flowers from the stems. If necessary, shake the flowers lightly to remove dirt, but make them not clean with water (then it loses flavour).
Put the umbels in a large pan or bucket. Then add water until the blossom is exactly submerged. The blossom tends to float. If necessary, put something heavy on top, such as a plate. That way, all the blossoms will remain under water. Put the pan or bucket away for 24 hours.
After 24 hours, pour the contents of the pan through a sieve (and collect it in a second pan). Discard the flowers. Repeat this process: then pour the water back into the other pan through a tea towel or hydrophilic cloth once more to get it really clear.
Put about 2 litres of this flower water in a pan, chop and add the lemon, add 1 kg of sugar. Optionally, you can also add mint, strawberries, rose petals, cinnamon, ginger and other spices to make syrup with a twist. My advice is: start with a good basic elderflower syrup first. Is it successful? Then possibly make another batch with a twist.
Heat the mixture. Make sure it does not boil or it will lose flavour. Keep stirring so that the sugar dissolves well. Also add citric acid at this stage and keep stirring.
Sterilise the fleesen (and don't forget the cap). Sterilise: pour boiling water over the bottle and then put the bottle in an oven at 100°C for 10 minutes.
Pour the hot syrup into the bottles with a funnel and seal them immediately. Let them cool down. Don't have a funnel? You can make your own funnel by cutting the bottom of a plastic soft drink bottle.
Store the bottles of syrup in the fridge. It is quite normal if you see sediment at the bottom of the bottle, this is pollen from the flowers.