When you select a Sales Order to display, you will see an "INVOICE" button at the bottom.
Any Invoices already created from this order will appear on a drop-down menu attached to that button. When you select one, the system will display a preview of that invoice and allow you to email it to your customer. There is also the option to create a new Invoice on the drop-down menu and when you select that, the system will generate and display a brand new Tax Invoice from your Order.
There is also a check box which allows you to automatically create an invoice, whenever you create or edit a Sales Order - when you hit "OK" to save that sale or quote, you will see a preview of the invoice which has been generated.
You can customise your Invoice by including a personal message in the Sales Order itself and this will appear on the Invoice. If there is any payment owing on the order, the system may ask you for your bank account details if it doesn't already have those on record - these are to put at the bottom of the invoice, so your customer knows how to make their payments to you.