Go to BOOKINGS and open a booking. If you want to add an invoice number to the booking you can use the field "referer" or any other filed you dont need e.g. a custom question, or fax (in this case you will have to adjust the template variable).
[[File:Invoice_7.png]]
If you want to add custom information just for this invoice click on the "edit" icon (2)
[[File:Invoice_7.png]]