Keep it positive. Remember that communication and open dialogue more often than not will lead to something positive. It is important to keep things calm and professional when talking to the customer.
You promote yourself through your work. It can be a good idea to accommodate the customer, especially if you can see the inadequacies that the customer presents. This can either be done by verbally acknowledging the inadequacies and apologize, but you can also accept a refund for some of the cleaning. This could for example be 1 hour out of a 4 hour booking.
Be open to feedback. The more open you are to understand the potential shortages of your cleaning, the better you will be off for future cleanings. The customer might have some fair points which, if listened to, will improve your work in the future.
No two customers are the same. You will experience that different customers handle their dissatisfaction in vastly different ways. Just like Helpers are different too. Remember to be patient and acknowledging throughout the conversation.
It also has to be fair to you. If you against expectations experience a customer who appears unfair and does not keep a positive tone, it is okay to stop the conversation and refer to HH support. (In a kind matter of course.)