Lightly pound the juniper berries in a mortar. Chop the juniper needles. Then sterilize a large screw-top jar (a good 1 liter content) in boiling water. Add the rock sugar, juniper berries and needles to the glass and mix. Pour the vodka on, seal the glass and let it steep for 4 weeks, shaking occasionally. Then strain the berries and needles. Fill the liqueur into sterile bottles and let it steep for another 6 weeks.