The steps.
Open Customers and add a customer with a name, contact details and billing address.
Add a VAT number or another supported customer reference when needed. Check the details before using them on a document.
Select the customer when preparing an invoice or estimate. For an existing list, use the customer CSV import in Settings → Tools.
Before you finish.
Check the CSV preview and import result. Updating a customer does not replace the need to review an existing document before sending.