Once you have a project with at least one project item, you can create an invoice. Right click on the project and choose Create Invoice. If you have enabled auto-numbering in the Billing section from Business » My Business Details, an invoice number will be automatically created for every invoice you create.

If you want to invoice multiple projects, right click on the client whose projects you want to invoice and choose Create Invoice.

When an invoice is created, the application automatically switches to the Invoices view. You can switch back to the projects view from View » Projects.