Hi,
To configure the preferred Document Type for Payment Lots, use the following configuration path :
Financial Accounting->Contract Accounts Receivable and Payable->
Business Transactions->Processing incoming and Outgoing payments->
Define default values for Payment Lot
There you can specify the desired Document Type (assuming it has already been defined as a FI-CA Document Type).
If you are new to FICA, I suggest you purchase the SAP Ebook for FICA configuration. It shows you the basics of FICA configuration, and is not too expensive.
Regards,
Ivor