If your Mac has TB3, you can just install Windows 10 directly to TB3 external drive without using Boot Camp partition. Windows installation treats TB3 as an internal drive during installation.
If you want to use USB, I suggest you use at least USB 3.1 gen 2 external drive for faster speed.
Easiest way to install Windows 10 on external USB drive is to pre-partition the external drive. And then using Windows Apply Image. If you need help with using Windows Apply Image, I can post how-to. I believe, there are many postings how to do this already on this forum.