Nearly every time I install Windows, it turns into a chicken and egg problem. I need a Windows machine to create a Windows USB, but I don’t have a Windows machine. So, to create a Windows USB from macOS, here’s the trick:
# Format USB drive with MBR and MS-DOS (FAT)
$ brew install wimlib
# Copy all files from the Windows ISO to the USB drive (except for the sources/install.wim file)
$ wimlib-imagex split /Volumes/YourISO/sources/install.wim /Volumes/YourUSB/sources/install.swm 4000
# Eject the USB drive