You can attach notes to a specific booking by using the notes field located to the right of the invoice (or below depending on your screen size). Add your note in the field provided (1) and then click the Create button (2) to finish.
If you also wish for the note to appear on the invoice, select Show on invoice. Note: this also appears on the customer invoice.
If you need to delete or edit a note, you can hover over it with your mouse and select the appropriate link.