In order to create a new invoice, you need to go to Accounting → Invoices → Add invoice. You can add items manually to the invoice by inserting new rows or by clicking on the Add outstanding time/costs and select from the dropdown:
NOTE: If you select Add from Timesheet, there's the option to attach a time report to the invoice to offer more details about what you've been working on.
You can also create an invoice by going to Projects → Select a project → Overview → Click on Invoice Unbilled AR.