For computers with Windows:
Install (or run) Rufus.
If you see an option that says Persistent partition size , then put in a good amount. For 32 GB USB's put 20 GB.
If you have a USB drive that is less than 32 GB then don't add the persistent partition.
Press START.
Once done you can either reboot to the USB or eject it.
If you want, here's a video to show you:
For computers with Linux:
Extract the ISO image to your USB drive. (Format it first!)
Once done, you can either reboot to the USB or eject it.