Question on how to design a form with multiple steps

I think this is a pretty common thing, but it's my first time doing it so I'm a bit lost. Basically I need to make a booking or an appointment. For that I need a customer to attach it too, so when the user first wants to create an appointment, they have to select a customer first, but if the customer doesn't exist, I want them to create a customer. I'm confused as to what should be on what page, should my 'create appointment' button go to select/create customer, which can the go to 'create customer' and back to 'create appointment', it seems clunky? I'm making this app for computer illiterate people so I want to the process to be smooth.

Please, any advice would be very much appreciated.
Thanks