How do I install Mac OS X El Capitan on my other iMac? I'm relatively new to Mac and I cannot for the life of me figure out why it is so complicated to download an OS installer, create a bootable disk and use it on another machine.

According to the official Apple documentation here, one must:


  El Capitan downloads as a disk image. On a Mac that is compatible with
  El Capitan, open the disk image and run the installer within, named
  InstallMacOSX.pkg. It installs an app named Install OS X El Capitan
  into your Applications folder. You will create the bootable installer
  from this app, not from the disk image or .pkg installer.


I downloaded the 6 GB file.
But when I run the installer, I get a message saying 


  This version of OS X 10.11 cannot be installed on this computer.


But I am not trying to install it on this computer! I want to install it on a different computer!

How do I just get the OS image so I can create the bootable drive to use in the other machine?

I really appreciate your advice!
Given that you have the IntallMacOSX.pkg file you need to extract the app -so, you want this answer: How can I open a .pkg file manually?

You can then extract the "Install OS X El Capitan.app" application into your Applications directory and proceed with the steps as outlined by Apple.