From the transaction dialogue, you can click on the "Direct Debits" button to navigate to the above screen where you can set various Direct Debit instalments as paid.
When all instalments for a particular invoice are marked as paid the invoice itself will be set as paid.

• When enabling the Direct Debit payment option for the first time, the system will create 2 new database columns, Direct Debit Account (TEXT-16) and Direct Debit Active (CHECKBOX).
  These fields are used to hold the member's bank account number and whether they have been approved to pay via direct debit.
• You will first need to discuss the direct debit facility with your own bank. They will need to approve your organisation for accepting payment via direct debit.
• Each member that wishes to pay via direct debit will need to complete a direct debit payment authorisation form (supplied by your bank).
  Once your organisation has accepted the member's authorisation form, their bank account number will need to be saved against their profile (Direct Debit Account Number) and you will also need to check the Direct Debit Active checkbox.
  You will also need to manually increment the member's expiry date to the end of the current membership year.
• Your bank will require that you submit a file containing all debit orders once a month.
  There is no automated way to submit DD payments to a bank. An import file is generated and required to be uploaded to the bank by the user.
  Expert Developments may need to create a custom app to create and export this file according to your bank's requirement.
• Direct Debit payment is only available for membership subscriptions.