First things first: don`t boil the flowers !!! Otherwise the taste will be very strangely earthy and not appetizing (I think).
To get the lilac taste (smell), everything but the sugar is put in a bowl and placed in the refrigerator or other cool place for at least 8 hours.
Then strain the whole thing through a cloth over the sieve, pour into a saucepan, add the preserving sugar and bring to the boil while stirring. Then let it boil for 5 minutes, stirring constantly, and immediately pour into clean glasses.