Existing layaways will be on the layaway tab on the terminal side of the software. To find the layaway to make a payment on search the customer name here
Click on the layaway details line (not the blue name, that will just take you to the customer's information.
Click Add Payment and type the correct payment method to how they are paying
The latest payment will apply and be shown on the top of the grid
You can hit the blue X in the top right to close out the layaway and you're done making a layaway payment.
More about Layaways: