How to install OS X El Capitan onto Mac through a USB when no previous OS is installed
If you have an El Capitan USB installer, the fix is very simple. Make sure the laptop is off. Plug the computer in, then hold the "Option" key while the computer starts up. Keep holding it until you see an image of a drive on the center of the screen. Hit the "Enter" key to boot from the flash drive. Then the computer will walk you through the steps to install El Capitan.

Let me know if these directions did not work for you or if you need more explanation of these directions.