I don't really understand how Pro Site runs

Hello,

I'm setting Pro Sites.
But after, I don't really understand how and where the client can choose the level he wants.
And after choosing the level, what happen how does he use it , how does he customize and create his website ?

Where can I find a demo website or a video who explain me the way of a client from the choice of the level to the creation of his website ?

Thanks
Pending your reply.
Ludovic