You will need to boot to the USB flash drive on the computer that you wish to use to run your custom ChromiumOS build. I will not cover the steps to boot to a USB drive, please refer to the specific manufacturer instructions if you are unfamiliar with the boot process. Furthermore the installation of the image onto the computer will destroy all of the data on the hard drive – be sure to backup anything you wish to keep.
You will see a GRUB Boot Menu, you do not need to select anything on this screen. You will eventually be taken to the ChromiumOS Welcome Wizard, do not attempt to sign in at this time. Press ALT+CTRL+F2 on your keyboard and you will be taken to a shell login. Enter the username “chronos” and the password is “test0000”
Run the following command (Replace the /dev/sda if needed). This will erase everything on the computer.
sudo /usr/sbin/chromeos-install --dst /dev/sda
You can now reboot your computer and if everything went well, boot into your custom build of ChromiumOS. After logging in with your GMail/GSuite account, you can enable support for Crostini by opening the Settings and selecting “Developers”.