This guide will show you how to install and configure elementaryOS Loki on your Acer C720 or C720p. I'm assuming you have installed some sort of Linux on your Chromebook before and already know your way around in Developer Mode. If not, read and follow https://www.chromium.org/chromium-os/developer-information-for-chrome-os-devices/acer-c720-chromebook sections "Developer Mode" and "Legacy Boot".
Nothing special here: Download the ISO from https://elementary.io (put in $0 as an amount to download the ISO for free, although you should consider donating)