How to make a bootable USB drive from Install OS X El Capitan.app?
(From OS X Daily)

You'll need an 8 GB or larger USB Drive. Remember, it will be erased, so backup any important files.

Use the following command in Terminal (assuming the Install OS X El Capitan app is in /Applications, which where the App Store installs it by default)

Also make note of the USB Volume Label and use its name in the command below in place of: &lt;USB_Volume_Name&gt;

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume "/Volumes/&lt;USB_Volume_Name&gt;" --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction