Prepare the file you already use.
Export a spreadsheet as CSV or use a JSON client list. A client name is the starting point; you can also include email, country, currency, payment terms, tax ID and address. The import screen shows the expected fields.
Review how to handle existing clients.
MostlySame can skip matching clients or update their details when you choose that option. Review the result so the list is ready for your next invoice run.
Use the imported clients in your invoices.
Choose a client while preparing a single invoice or a group. Combine the saved details with your products and templates, then check the invoice before issuing it.
Keep the list useful over time.
Update a client directly when their details change or use another import for a larger update. Already-issued invoice documents keep the client details they were issued with.