Fill a wide-mouth, sterile glass jar with clean pine needles. Use any size of jar, just be sure to pack the needles firmly inside.
Pour room-temperature, pasteurized apple cider vinegar over the needles until they are covered. Boil the vinegar and bring it back down to room temperature before you pour it over the needles if you are concerned about mold or other organisms growing in the jar. Turn the heat off as soon as the vinegar comes to the boil.
Place a plastic or otherwise non-metal lid on the jar. Seal it tightly and label the jar's contents and the date it was created. Store the vinegar in a dark room for at least six weeks before you use it.