Before creating an ISO file, you need the official installer. The most secure method is via Apple's Support Website.
Disclaimer: This guide is for educational purposes. Always use official, secure sources for software downloads. Mac Os X El Capitan 10.11 6 Iso Download
Inside the mounted image, open the InstallMacOSX.pkg package installer. Before creating an ISO file, you need the official installer
hdiutil attach /tmp/ElCapitan.dmg -noverify -mountpoint /Volumes/install_build Restore the installer to the image: Before creating an ISO file
: If your Mac is compatible and you have previously "purchased" it, you might find it in your Purchased tab.