Seems like every two years I have a post like this, but here is an update on how to do this:

1. Ignore the guides that talk about unetbootin, this does not work on High Sierra nor did it work on Sierra. When I tried to use it with a Windows ISO, it did create a bootable drive, but there was no option for Windows in it.
2. Ignore the guides that talk about using Bootcamp Assistant to do this, in High Sierra, they removed this option and it appears that you cannot create a bootable this way.
3. This guide also does not work. Using a Disk Utility format from the command line with diskutil eraseDisk MS-DOS _Volume Name_ MBR disk_n_ where disk n you get from looking at diskutil list for the disk number. Then apparently you can just unix copy all the files from an ISO. So first mount the ISO and then do a cp -r