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:
Add from Timesheet – creates an invoice based on tracked time
Add from Expenses – adds expenses to the invoice, but make sure they have the same currency as the invoices.
Add from Projects – creates an invoice based on flat rate projects
Add from Tasks – creates an invoice based on flat rate tasks
If you select Add from Timesheet you also have the option to attach a time report to the invoice to offer more details about what you've been working on.