Mount the .dmg file, install the app, and use the createinstallmedia command in Terminal. Format an 8GB+ USB drive as Mac OS Extended (Journaled) named “MyVolume”, then run:
/Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app