How to get Ubuntu (Linux) on Mac OS X / macOS (Dual Boot)

How to get Ubuntu (Linux) on a Mac running macOS or Mac OS X. This process is called a dual boot. Links: My website to get the bootable drive maker: Ubuntu ISO: Command to run in Ubuntu to install rEFInd: sudo add-apt-repository ppa:rodsmith/refind && sudo apt update && sudo apt install refind && sudo refind-mkdefault