Go to the Project under which you would like to create the Invoice.
Under the Money horizontal navigation, select the Invoices sub nav item.
Click on the New Invoice Button
You’ll need to add line items or additional Groups. If you would like to just keep to using a single group then simply click on the Add Line Item under the first group that has already been created for you.
If you would like to add another Group then click on the Add Group.
Note: Groups existing for two fundamental reasons. Firstly, they make things easier for clients to read. And secondly, you can create a Milestone from a Group when you create Tasks.