Easiest way is to create the Windows 10 installer on the USB using the Microsoft tool for it (or download the iso and burn it to the drive but you can't just paste it there).
Then with the USB drive attached, you should be able to boot while holding the Option key until you see the available boot devices. Choose the USB drive and follow the instructions.
If you're not keeping macOS installed you can format the whole drive. Otherwise format the drive first in macOS before you start, although sometimes Windows doesn't like being installed on the same physical drive as another OS these days.