This document sets out how I installed GalliumOS 2.1 on a new Acer Chromebook 14" (CB3-431) 'Edgar' in April 2017. I installed GalliumOS on the internal eMMC storage, but left Chrome OS in place (allowing dual-booting).
It is meant to be an easy-to-follow and particularly thorough (if repetitive) guide, but I make no warranty that it will work correctly for you. It will wipe all data on your Edgar.
I have tried to provide references for each section; see the GalliumOS wiki guide to chrx installation for an overview of the general process.
⚠ Caution: There have been reports of Edgars' speakers overheating, due to a malfunction of the audio hardware, when booted into anything other than Chrome OS. I have not experienced this, and it seems to be believed that this will not now occur under GalliumOS. More information can be found in the comments for the [GalliumOS Braswell Platform Validation