Put the rinsed and well drained spruce tips in a large, wide lid jar. Sprinkle the sugar on top and pour in the rum. Mix everything well and close with the lid.
Let it mature for six months. Stir once a week during this time. In the last 14 days, add sugar if you like, if you like.
Strain the liqueur, pour it into a bottle and enjoy.
My approach has only recently matured, but it already smells very much like tart pine needles.