In addition to this approach, payments can also be added in Client->Payments tab.
Right-click on the document you want to add payments for to reveal the context menu.
1- Set payment as incoming or outgoing.
2- Select a payment type.
3- Set payment amount
TOTALS will automatically fill in the fields for 1-3. Usually you don't have to change them.
4- Optional slip or description
5- Payment date