The process in El Capitan is (compared with root-canal work, income taxes, or previous versions of OS X) relatively painless. How to install (or reinstall) OS X. In theory, you should have to install El Capitan only once, or never if your Mac came with El Capitan preinstalled. And in a perfect world, that would be the case.

If you use pkgutil to expand the InstallMacOSX.pkg it opens to three items, the Distribution file, InstallMacOSX.pkg and Resources. You can alter the Distribution file to ignore the supported platform check, see below.

Creating the Install OS X El Capitan.app on a mac that came with a newer OS than El Capitan.

Download the InstallMacOSX.dmg from Section 4 of How To Upgrade To El Capitan

Double-click to open InstallMacOSX.dmg to Install MacOSX.pkg.

Drag InstallMacOSX.pkg to the Desktop.

Open Terminal, enter the text

Boot Disk Utility El Capitan

A folder will be created on the Desktop after a while with the contents of the .pkg file.

In there you will see an item called Distribution, Control-click on that and select Open With

go to Other and select Text Edit.

When the file opens scroll down to the section that reads,

Free Ms Dos Startup Disk

function isSupportedPlatform(){

type in at the end return true; so it now reads as,

function isSupportedPlatform(){return true;

Now click Save and close the document.

Go back to Terminal and enter the text,

press Return, after several minutes, be patient, (check to see if the prompt has returned to Terminal) a new package will be created on the Desktop.

When created double-click on that and an installation window will open where you will now convert the

new InstallElCapitan.pkg to the Install OS X El Capitan.app which will be placed in your Applications folder.

You can now create a bootable USB using the install app and createinstallmedia,

If you have the time I would be grateful if you could give this a trial, it works here on my mac, but I would like confirmation from yourself or any others reading this that the procedure works on macs that came with macOS Sierra or newer pre-installed. Oh and tell me if my description of the procedure is easy to follow and understand..