This is the safest and most reliable method. You will need a working Mac (any version from Snow Leopard to Monterey) or a macOS virtual machine to start.
This article will walk you through legitimate methods to obtain a clean El Capitan 10.11.1 installer, convert it into a bootable ISO image, and ensure it works for your intended purpose – whether burning to DVD, writing to USB, or mounting in a virtual machine. macosxelcapitan10111imageiso work