Before answering your question, I'd suggest that you don't use it because the invoicing function is nothing more than a glorified word processor.  Also, whatever you type in won't get saved anywhere.  Once you produce the PDF file with it, you'd better save it somewhere because it'll be gone, poof!  And if you want to modify anything on that invoice later, the only way is to re-create it.

You are much better off using OBOA or other invoicing solutions.

Back to your question, if you must...

1. Click on the Tax Returns dashboard and select the Year you would like to bill;
2. On the line of the return for which you would like to bill, click on the dropdown under ACTIONS and select Invoice;
3. Make your entries and hit Create PDF when finished.