How can I create a USB installer of OS X - El Capitan on a computer that is running macOS Sierra? I've downloaded a copy of the OS X - El Capitan Installer, which I want to make a bootable USB of.

However, it requires you to run the installer on whatever machine you are running, and installs it as an app. 

But I cannot run the installer (to create the app) because I am using the computer running macOS Sierra.

It just gives an error "this installer is too old to run on this computer."
You can use the createinstallmedia command. This link explains: https://support.apple.com/en-us/HT201372