Collect only fresh flower heads and make sure the tiny buds have just opened and use them the same day. This is important because the lovely fragrance will turn into bitterness if you wait too long. Do not wash the flower heads because you will lose too much flavour.
Make sure to look through them carefully and remove any insects. While doing so make sure to remove unopened buds and any flowers that are not perfectly fresh. They will otherwise spoil the taste. When you are done with this, the most labour intensive task is done.
Zest the lemons and slice them. Don't discard anything; all will be needed.If you do not have a lemon zester, then peel them carefully with a vegetable peeler.
Place the flowers with the citric acid and zested lemon slices with their peel in a large pot with lid.
In a second pot, bring the water and sugar to a boil and remove from the hot stove top. Pour the sugary, hot water over the flower mix and give it a stir. Cover with a tight fitting lid or plate and leave to steep over night.
The next day strains the cordial through a fine mesh sieve or cheesecloth. Discard the flower and lemon mix. Fill the cordial into sterilised bottles or jars and store in a dark and cold place.