Fill a jar 3/4 full with fresh mugwort leaves, or half full with dry mugwort leaves. Pour apple cider vinegar over the herbs, leaving a half inch or so of headspace in the top of the jar.
Cap with a tight fitting, non metal lid. If you are using a metal lid, place a piece of wax paper under it to help prevent corrosion.
Let the vinegar steep for 2–4 weeks then strain through a cheese cloth and rebottle.