Multiple themes during redesign

I’m working on a complete redo of my website with new content and formatting as well as a new theme I’ve purchased. The overhaul is quite expansive so I don’t want to switch the theme outright until pages are laid out and ready but am having trouble customizing and building pages somewhere they can be saved as drafts until the website is ready.
Does anyone know how to do this? Preferably with detailed steps, I’ve been told to make a subdomain but don’t know how to do that either so need help.
Thanks!