According to The Packet (patterns, articles & essays for the 18th century reenactor) by Mark Tully, spruce beer was often issued to troops as a substitute for the daily rum ration. It was thought be beneficial to the soldiers' health, and was considered especially good for warding off scurvy.

There is an 18th century recipe included for one gallon of spruce beer (if you wish to make more, adjust the recipe accordingly).
Tully warns that the final product is quite potent and tastes much like an English stout (if you use your imagination -- sort of). It goes flat rather easily, so cork it well once the yeast is done doing its work.

Tully warns to stay away from hemlock spruce, and do not use the family Christmas tree either!

Okay, here we go....

SPRUCE BEER (per gallon)
12 oz. molasses
1/2 oz. yeast (active dry yeast works well)
1/4 lb. fresh spruce tips (or 1/2 oz. extract)
1 gallon water

Break up branches into small pieces and place in water. Bring to a full, rolling boil, then simmer for two or three hours. Add more water occasionally as it boils off and skim off any foam that forms. Next, strain off the liquid and add molasses to it. Bring the mixture back to a boil again, then let it simmer for another 20 - 30 minutes. Skim as necessary. Let the brew cool until "milk-warm" (98-100 degrees) and stir in your yeast. Let stand loosely covered for several days or until the yeast quits working. Then bottle or barrel up the beer and cork tightly until ready for use.

Since there were limited refridgeration capabilities in the period, the soldiers would have probably had this at room temperature (65-70 degrees), but you could chill your spruce beer to make it more palatable if desired.

CHEERS!