Take the leather item outside and brush off loose mildew with a soft, dry brush
Wear gloves and a mask
Mix equal parts white vinegar and water
Dampen a soft cloth with the solution
Wipe the mildew spots gently
Use a clean dry cloth to remove moisture
Let the leather air-dry completely away from direct heat or sunlight
Wipe the leather with a leather cleaner if needed
Apply a leather conditioner after drying
Repeat if mildew remains
Store the leather in a cool, dry, well-ventilated place