How do I cash in the total credit’s of my customer on his account?

You simply have to go to the customer account in question, and click on the debit amount or Balance, to display the details of the order. Then you click on "Credit the Account" on the right bottom.

Once you have clicked on "Credit Account" you will have this following screen, in which you will have to type the amount that your client is paying to you, and then you have to select the payment method he is using.

You will also be able to print a summary ticket of this order which you will be able to give to the customer.