Adding a new payment that can then be allocated to an invoice, for either a customer or supplier, is covered in the video below. A step by step guide can be found below the video.
Steps
Open the Customer/Supplier record.
Go to the Statement tab.
Use the Actions menu to select New Payment.
In the Payment Dialog
Select the Payment Type.
Select the Currency.
Select the Bank Account.
Select the Payment Date.
Enter a code (e.g. cheque number).
Select the Payment Amount.
Click OK.
Check that the payment appears in the Allocate From grid.