Ok, let's make sure you do it exactly this way.
On a functioning Windows computer, download the Windows Media Creation tool here: https://www.microsoft.com/en-us/software-downlo... Please make sure you download it to the computer, not the USB. Then you need a flash drive that's at least 8GB. It is a flash drive, NOT an external drive with USB wire. Please also make sure there's nothing in the drive.
Then you run the tool you downloaded, and choose that you want to make an installation media tool, and follow the rest of the steps to have it download the Windows installation files into your USB.
Once you have the flash drive made, you can go to your computer's boot option to have it boot from the USB. And then choose Install option > Custom Install. When asked where to install, delete all of the partitions until they merge into one unallocated space, then choose that space and continue.