There are many ways and many different tips saying this or that is the right way. Some may fit for you others not, you have to decide which way you want to go, but I really like the following statement from a topic about commissions:
Be bold, they want you to work for them, to do a piece you often can not offer to other customers because for them a portrait of person or pet XY is worthless. They ask you, because they think you are the one who can do it for them, they selected you because of that. So what to do? I would ask for the full refund beforehand! If they believe in you and your capabilities and don’t want to cheat they’ll pay you beforehand, if not they are probably “problematic” customers.
Michelist