First take some drops of liquid or do some spray on the shoes then clean it and gently scrub it with the soft cloth or with the towel but do not use brush or hard thing to scrub it. Do not rub abruptly scrub slowly. After that let it dry and minimum half an hour you will see that the sot will be vanished and the Ugg Boots are shining and remember one thing do not dry these shoes in the direct heat of sun. But if you are using cleaner or conditioner then do not apply it directly because it will be harmful to your Ugg Boots. So ever used dilute cleaner and conditioners. Use hot water rather than cold one.