I am installing on an HP Chromebook,
Chrome OS Version 78.0.3904.106 (Official Build) (64-bit)
Linux-Debian 9.11 (as per cat /etc/debian_version which is the same Debian that Ubuntu 17 “artful” uses sometimes, so that’s the Seamly install I will be using)
These directions might work with any Chrome OS after 69, previous ones don’t have convenient Linux activation.
Step 1: Activate Linux (Beta) in Settings
Step 2: Wait 300MB.
Step 3: Open Terminal
If you know how to use vim you’re probably not reading this for education, but for entertainment &/ review. You’ll know what you can do differently.
Step 4: Enter sudo apt install nano
Step 5: Enter sudo apt edit-sources
You will see that the current sources are debian, which is what Ubuntu is based off of also, so it might be a compatible architecture, yes?
Step 6: Down-arrow down to the line under the existing entries.
Step 7: Enter
deb http://ppa.launchpad.net/susan-spencer/seamly2d/ubuntu artful main
deb-src http://ppa.launchpad.net/susan-spencer/seamly2d/ubuntu artful main
Note that the first three letters of both of the separate lines are “deb”
Edit note: I originally used the “trusty” install, but further exploration revealed that “artful” was probably closer. A few minutes doodling & it seems more stable.
Step 8: <ctrl>x to exit nano
Step 8.5: y <enter> to save on your way out.
You will be asked to run apt-get update.
ETA: run curl -s "https://keyserver.ubuntu.com/pks/lookup?op=get&search=0x798d7516047d09ac2446967b807da747977ace23" | sudo apt-key add -
Step 9: Enter sudo apt-get update
Entering the command string in the “ETA” line nullifies the blurred paragraph below. I would like to thank the syncthing download page administrator for the inspiration.
It will complain about not being signed. HOPEFULLY SOMEONE CAN TELL ME HOW TO GET IT SIGNED This will not make it unusable, just potentially dangerous. I assume that the “Signing key” listed on the Ubuntu Seamly install page has to be entered somewhere.
Step 10: Enter sudo apt install seamly2d
Step 10.5: When it asks if you want to “Install these packages without verification? [y/N]” Enter ‘y’
Step 11: When your command prompt returns, you’re done!! Feel free to exit the Terminal, navigate to ‘Linux apps’ in the App list, & click ‘Seamly2D’ to run.
tada! you did it! breathe a huge sigh of accomplishment, & make sure to save constantly, because this isn’t very stable. When it freezes you don’t necessarily need to reboot your chromulent computer, just navigate to the Terminal, run top, identify seamly2d near the bottom of the list, press ‘k’ (for Kill,) enter the PID which appears on seamly2d’s line, enter kill, go reload Seamly2D! (I also like pinning Seamly2D to my dock)
Note: After switching to the “artful” install, a few minutes doodling seems reasonably stable, but I did manage to freeze it while trying to layout my foot long doodle on a letter sized field. I seem to recall reading around here somewhere not to do that. Such are the risks of installing on an unsupported architecture! 
Is it worth it? Absolutely.
Maybe I’ll try compiling it for Chrome Linux by hand later, I figure that will be more stable. After I pull that off successfully, I might even try crazy stuff!
(For those who wonder, the Chromium OS docs appear to be found at Chromium OS I didn’t bother looking carefully at it for this thing though.)