Booting an installation medium on a Mac requires some additional steps, especially if you don't have an Apple keyboard.

Install Linux on a Mac.md

Boot from USB on a Mac

This short snippet explains how to boot from an installation medium on a Mac. The installation process itself is extensively documented for most operating systems. However most modern Macs require some additional steps to even get to the point of running a live DVD, especially if you do not have access to an original Apple keyboard. This snippet documents this process.

Create an installation medium

This process is well documented. One way of creating a bootable medium is to use balenaEtcher. Install via Homebrew:

brew cask install balenaetcher

Use it to create a bootable medium from an ISO.

Enter Recovery Mode

Boot into recovery mode by pressing ⌘R on boot. Note that this only works reliably with Apple keyboards. Alternatively you can run

sudo nvram "recovery-boot-mode=unused"
sudo reboot

The recovery mode flag is usually removed automatically but can be unset manually via nvram -d recovery-boot-mode or by resetting the NVRAM. [Source]

Disable System Integrity Protection [Source]

In Recovery Mode open the terminal and run

csrutil disable

Then reboot. You may reboot directly into the installation media if possible.

Boot the Installation Medium [Source]

On boot hold the ⌥ key to get into the boot menu. If you are using a non-apple keyboard you can instead run:

sudo bless -mount /Volumes/Startup_Disk_Name -setBoot
sudo reboot