After you have set up and saved your invoice settings, the fastest and easiest way to issue an invoice is directly inside the Calendar:

In the Calendar, click on the reservation for which you want to issue an invoice.

When a small window pops up, on the left side click on Invoice. 

An invoice with all items is displayed. Now you can edit existing items or add new ones. 

If you save an invoice as a Draft, you can edit, change or delete it afterwards.

When an invoice is issued, it is not possible to change it further. You can only cancel it and create a new one if you need to make additional changes. 

When previewing a draft or an issued invoice, you can immediately print it or save it as a PDF file:
​

You can find your issued, canceled or drafted invoices and everything else related to invoices by clicking on Invoices in the main menu:
​