COURTESY : http://mashable.com/2015/10/01/clean-install-os-x-el-capitan/#3ktuA0zxhEqQ
If you want to do a clean install of OS X El Capitan on your Mac — or on a new hard drive — you can do it, it just takes a bit of work.
If you’ve gone through the clean install processes in the last few years, these instructions will look familiar. (That’s because they are almost the same as past years, albeit with changes made for the proper OS name). With a little time, it’s relatively easy to create a bootable copy of OS X El Capitan that you can run from a USB stick or other drive.
Note: A clean install will erase all of the contents on your disk drive. Make sure to back up your important files, settings and apps before proceeding.
Download OS X El Capitan
Format USB Drive
Easy Option: Use DiskMaker X 5
Option 2: Using Terminal
Booting up and Installing El Capitan
After the Install
After installing OS X El Capitan, you can start restoring your programs and settings using either a Time Machine backup or Migration Assistant.