Apple has actually included a terminal command that can create an install disk for you. Assuming that you have the OS X El Capitan installer in your Applications folder and you have a Mac OS X Extended (Journaled)-formatted USB drive named "Untitled" mounted on the system, you can create an El Capitan install drive by typing the following command into the Terminal.
sudo /Applications/Install\ OS\ X\ El\
Capitan.app/Contents/Resources/createinstallmedia --volume
/Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ El\
Capitan.app --nointeraction
The command will erase the disk and copy the install files over. Give
it some time, and your volume will soon be loaded up with not just the
OS X installer but also an external recovery partition that may come in
handy if your hard drive dies and you're away from an Internet
connection.