Downgrading from Hi Sierra to El Capitan

Hi all,

I'm hoping that someone has the time to help me. I am ' simply ' trying to downgrade to El Capitan and I have spent all week in vain. I'm so tired :-(.

I have tried creating a bootable USB and that wasn't a success and I have also done the old SHIFT/OPTION/COMMAND R at power up to take me to Disk Utility to erase the Macintosh HD and try to re-install the original os, so as I can got from there but it keeps reverting to High Sierra.

Is there anyone out there that would be able to help please ?

Most grateful

Al ;-)