How Can I Add a Payment Method to a Specific Payment in AR and AP Modules?

In order to add a Payment Method in  the Accounts Receivable module, you need to navigate within the "Manage Payments" section on the three-dot menu:

Once you are there, you can add a payment and choose the desired Payment Method:

For the Accounts Payable module, the procedure to add a Payment Method is similar. Simply head to the Manage Payments option: 

Within this option, add a payment and choose the Payment Method:

In case a new Payment Method is needed, it can be added by a user with Admin access in Administration > Payment Method > Create Payment Method