Take the leather item outdoors and wear gloves and a mask
Brush off loose mildew with a soft dry brush
Wipe the surface with a dry microfiber cloth
Mix equal parts white vinegar and water
Dampen a clean cloth with the solution
Test the solution on a hidden spot first
Wipe the mildew-affected areas gently
Dry the leather immediately with a clean towel
Let the item air-dry in a well-ventilated shaded area
Apply a leather conditioner after the leather is fully dry
Repeat if needed for stubborn mildew
Keep the leather away from direct heat and sunlight
Store leather in a cool, dry place with good airflow