After a customer signs their mandate, you'll see a notification on your dashboard: "1 customer needs to be connected to Xero" (or QuickBooks/Sage).
Click Link now. This takes you to the Accounting Links page.
You'll see your NRTH customer listed with their name, address, email, currency, and masked bank account number. Next to them is a dropdown labelled Select contact.
Click the dropdown. It pulls directly from your accounting software's contact list. Search for or select the matching contact.
Once selected, click Link Contacts.
You'll see a confirmation: "Successfully linked 1 customers."
That's it. From now on, any invoice you raise for that contact in your accounting software will be collected automatically through their Direct Debit mandate.