If you want to install Windows 10 on to a Mac to use via Boot Camp then using the Boot Camp Assistant is the best option. Some newer Macs can do this without needing any external boot drive e.g. USB stick and purely by copying the ISO in to the Windows partition it creates. See Boot Camp for Mac: Install Windows on your Mac without using a USB hard drive or flash drive
If your Mac does not support this new feature it should offer the option to create a USB install drive which needs to be a USB2 drive of 8GB or greater in size.
If however you want to create a bootable Windows USB installer for use on a real PC not a Mac then Boot Camp Assistant is not suitable. In this case you maybe able to do this using the free Unetbootin tool. See UNetbootin - Homepage and Downloads
This tool is mainly for creating Linux install drives but apparently can also be used with a Windows 10 ISO image to make a Windows 10 install drive. See http://fgimian.github.io/blog/2016/03/12/installing-windows-10-on-a-mac-without- bootcamp/
Not all PCs support booting from USB and some may need their BIOS settings adjusted to enable this. Potentially you could burn the Windows 10 ISO to a DVD disc and boot that on the real PC.
