Site setup walkthrough from a customer perspective

I have set up a multisite that has three payment levels and no free blogs. I'm using Pro Sites and Pay to Blog (as instructed in other threads.)

I am struggling to see how a CUSTOMER goes through the process to access the payment grid and move to purchasing.

Can someone please give me a simple step-by-step walkthrough, page by page, of where a customer goes and what s/he does to do this?

Much appreciated.