Usually a Project will already have at least one invoice associated with it at the time of completion (when the Project is Closed).
For example, a Prepayment Invoice will have been created for 50% of the quoted amount at the start of the Project, or part of the finished jobs will have been Part Invoiced already. See previous article on Invoicing Active Projects for more details about this workflow.
Otherwise, a Project is fully invoiced once it is closed (when turning off the Active button).
Find it in the Closed Projects And Done Media Orders pane of the Invoice Creator, choose it and select Create Invoices (Selection:x) from the drop-down menu or via a right-mouse click.
Next, confirm both messages: "Confirm Creating Invoices" and "Confirm Changing The Projects/Media Orders To Invoiced".
The new Invoice will then be added to the New Invoices pane and the closed Project will disappear from the upper pane. When you search for the Project in the Toolbox, you will see that the status has been changed to Invoiced as well.