First step: Use baking soda
Fill the washing machine with hot water and add 100-120g baking soda, then wait for 30-60mins.
Second step: Use vinegar to kill the bacteria
Add hot water again and add about 300ml vinegar then wait for 30-60mins.
Third: Use a towel or toothbrush
Lastly, use a towel or toothbrush soaked in some vinegar and clean the surface of the washing machine.