You will only be able to generate invoices for appointments which have already happened. You can generate an invoice directly from an appointment by clicking on the appointment. This will open a window on the right hand side as below:
Click "Create invoice" as circled above. All the relevant details including this appointment will already be populated. You can click the blue "Save" button in the top left hand corner as below:
Once you click "Save" you will be directed to the page below where you will see a list of all invoices for this patient:
To learn how to record payment on an invoice, click here.