Here is a useful guide on how to obtain the installer, convert it into an ISO, and make it usable.
Open Terminal (found in Applications > Utilities). macosxelcapitan10111imageiso work
Once you have the legitimate El Capitan 10.11.1 installer (found inside the downloaded InstallMacOSXElCapitan.dmg or extracted from the .pkg ), follow these steps to convert it into a working ISO. Here is a useful guide on how to
Eli imagined the file as a tiny island of stability, an iso of El Capitan frozen in time. He could almost see the installer’s progress bar, that stubborn green line advancing toward possibility. He burned the image to a USB, not with the clumsy rituals of old but with a sleek command that whispered itself across the terminal: convert it into an ISO