For the juniper syrup, first crush the juniper berries in a mortar so that the berries burst open slightly. Pour boiling water over them in a glass jar, cover the jar and let stand overnight.
The next day, strain the liquid through a hair sieve and place in a saucepan with the brown sugar. Boil the syrup down on low heat for about 30 minutes.
Pour the juniper syrup into sterile bottles.