Wait for the mullein plants to begin to grow flowers. The best time to harvest mullein is right before the flowers bloom.
Cut the plant at the surface of the soil with gardening shears.
Tie string around the stem of the plant. Hang the plant upside down in a dark, dry area. You can do this either inside or outside. But if you do it outside, choose a spot with constant shade.
Let the plant hang until the leaves become dry and crumble easily. This can take one to three weeks.