How do I set up a staging subdomain to use as testing area?

[_Lance_]

Hi, 

Please advise me on how to create a staging subdomain that can be used to install a fresh, unedited theme and for a programmer/developer to run tests to highlight all the issues that are currently being experienced on the main store front on the domain. 

So to put simpler. 

https://www.staging.mydomain.com 

then upload a stock standard Theme, without any addons, apps, etc.

then have my hosting company create the CNAMES and DNS records 

and then let the programmer fix what needs fixing.

Why??

GMC Account suspension for website improvement policy violation, is my ongoing nemesis. and the more I try to "improve" the worse it has become, Little to no traffic, bounce rate has literally killed whatever traction the site was building prior to this. 3 faile

Anyway, the staging domain will be purely for test and then if that all works out, to merely use that working, functional property as the blueprint for the main domain. ( I hope this all makes sense)