Staging and making site live

[DAbrahams]

Hi,
I am pretty new to web design but am starting to build sites for myself and others.
I am looking for some help/support/advice on how to best setup staging sites on my existing domain.
So while I am building these sites I can give to staging address to the person that I am building for so that they can see the progress of the site and suggest changes.
i.e:
www.site1.mydomain.com
www.site2.mydomain.com
www.site2.mydomain.com
I think the correct way to do this is through Cpanel using subdomains plus all my sites will be wordpress sites so I also have to create the databases and users using phpAdmin..... I think...... Can anyway help on the full process and the best way to do this?

Also once I have finished making the site what is best way to replace the old site.... so I would have to move the new site from my server to their server replacing the old website.

I hope all of that made sense and I would really appreciate the help.

Thanks in advance