Edit: The quick answer to the question by a reader of my blog, Julien:
The info to download Chrome OS Flex from Linux is a bit hidden, but official info and link is available here: https://support.google.com/chromeosflex/answer/11543105?hl=en#zippy=%2Chow-do-i-create-a-chromeos-flex-usb-installer-on-linux
My dad has an Acer Chromebook 14 CB3-431, codenamed Edgar. Google just stopped supporting it with ChromeOS, but it’s still working well. Luckily, Google also just released the first stable version of ChromeOS Flex.
I decided to install the full UEFI image to the Chromebook from https://mrchromebox.tech/ so that starting Flex would be as easy as possible. That went well after finding and removing the write protect screw.
But it wasn’t too easy to find the URL to download the current ChromeOS Flex installation image. Google’s Chromebook recovery extension for Chrome does not work on Linux. By reading through some reddit threads, I found out that you can get the download URLs from this json file: https://dl.google.com/dl/edgedl/chromeos/recovery/cloudready_recovery2.json So as of this writing, the current image is https://dl.google.com/dl/edgedl/chromeos/recovery/chromeos_14816.99.0_reven_recovery_stable-channel_mp-v2.bin.zip
dd to write the image straight to a USB stick (not to a partition) and you should be good to go. Flex installs pretty much like a regular Linux distribution and seems to work well.