How do I install El Capitan on a I Mac that the OS X has been erease

Depending on the Age of the computer and IF it supports " Internet Recovery Mode " one maybe able to install an OS.

Connect via an Ethernet Cable for best results and restart the computer using any of the below methods

• On an Intel-based Mac, you can use Shift-Option-Command-R during startup to be offered the macOS that came with your Mac, or the closest version still available. Or you can use Option-Command-R during startup to be offered either the latest macOS that is compatible with your Mac, or in some cases the macOS that came with your Mac or the closest version still available.

If your Mac starts up to a question mark

A folder with a flashing question mark means that your startup disk is no longer available or doesn't contain a working Mac operating system.

If the question mark appears only momentarily before your Mac starts up normally:

1. Reset NVRAM.
2. Make sure that your preferred startup disk is selected in Startup Disk preferences.

If the question mark persists, follow these steps:

1. Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
2. Turn on your Mac and immediately press and hold Command (⌘) and R to start up from macOS Recovery.
3. While in macOS Recovery, use Disk Utility to repair your startup disk.
4. If Disk Utility found no errors or repaired all errors, reinstall macOS.
5. If you still need help, please contact Apple Support.

i tried these and they still did not work. after a minute or so the flashing grey folder with the question mark in it started flashing