I need help with downgrading !

I updated my iMac 21.5 late 2012 to macOS Sequoia. So it is much too slow, which I could have known. So I want to downgrade it to Catelina which I once had on it. I have no backup. Can someone give me a simple and step-by-step explanation on how to put it back on?

Author: @Mountain_Teacher_461