To create an invoice you must go to the person's profile first by clicking onto their name or searching for them via the search bar.
As long as you have entries applied from charges or refunds or financial movements on their account, you can produce an invoice by clicking "Actions" and then "Create Invoice"
The first box in the upper right is for selecting whether or not you wish to send the invoice to the postbag, or whether you do not wish to send this invoice (which will keep it safe in the invoices section instead)
Tick the boxes against all of the entries you wish to include on the invoice itself - or tick the top box at the column headers to tick all boxes on the page in one go.
Click "Create Invoice" at the bottom of the page.
You will see a display all of the uninvoiced transactions in a listed format - to tidy up entries please follow this guide which explains how to do this.