I would be wary with the 'average' income of 40k a month for marriage extensions. Jomtien Immigration at least, will only except 'Average' if you are working in Thailand and using tax receipts as proof.
If your sending it from abroad, 'average' isn't going to work with them.
You apply for the visa. Once it has been approved, you will then receive a notification for an appointment too attend the Embassy. You cannot do anything without the appointment. It would be unwise to book any flights as the visa may not be approved or they may require more documentation.