The official direct download for Mac OS X El Capitan 10.11.6
Downloading the DMG is only step one. You need a bootable USB installer. Here is the definitive workflow for the 10.11.6 DMG: Mac Os X El Capitan 10.11.6 Installer Dmg Download
Apple still provides the installer for older operating systems via their support pages. You can download the InstallMacOSX.dmg directly from Apple: The official direct download for Mac OS X El Capitan 10
: At least 2 GB of RAM (4 GB recommended for better performance). Mac Os X El Capitan 10.11.6 Installer Dmg Download