Formatting/Cleaning boot SSD for re-installation?

[Saiyan]

I have a Windows 8.1 OEM key that I've only used once on my new PC. I put it on a USB flash drive to install on the SSD. I ended up trying to delete some folder paths in regedit because I didn't know you could just hide them (the 'This PC' folders including paths such as pictures, videos, documents, and music) with regedit instead of deleting them. I want to do a clean re-installation of Windows 8.1 on my SSD. Is this possible now that I've already upgraded to Windows 10? If so I could use some help on how to format the drive so I can re install Windows 8.1 off the USB flash drive and start fresh. I know how to re format my HDD once I have the new O.S booted up but the only thing I'm not sure about how to do correctly is the formatting of the boot SSD drive I'm currently running Windows 10 off of. Any help would be greatly appreciated. Now that I know you can just hide the names of the folders it's basically the same as deleting them because I don't need them because I just made paths in quick access folders straight to my HDD.