Mix the water, mugwort leaves, and brown sugar in a large, non-reactive pot. Cut and squeeze the lemons into the pot. Bring the solution to a boil; let it boil for 30 minutes.
Cool the liquid to 70° F (21°C). To speed up the cooling process, you may need a larger pot of cold water to place the pot into.
When the liquid has reached the fixed temperature, strain it into a sanitized glass gallon jug lining your bottling funnel.
Pour the yeast onto the top of the brew in the jug, swirl the jug to mix the yeast into your brew. Let it fermented for about 10 days.
Siphon the beer into another sanitized gallon jar with 1/2 tsp brown sugar for carbonation. Close the bottles and store somewhere not too hot, the beer will be perfect to drink in 3 to 4 weeks.