To access this service, you will need an X11 server on your computer. This can be accomplished with any non-Wayland Linux machine, MacOS with "XQuartz" running, or Windows (somehow I'm sure).
Enable X11 to accept TCP connections. On MacOS, that's accomplished by running "defaults write org.xquartz.X11 nolisten_tcp -boolean false" before starting XQuartz. Linux seems a little harder, here might be a decent place to start
Make sure port 6000 on your machine is accessible to the internet, likely through port forwarding on your router. Ask someone who's setup a Minecraft server for help.
Run "xhost +" in an xterm. WARNING: this allows unauthenticated connections to your computer from the internet (though only display calls) and should NOT be left on. I tried something slightly more secure but couldn't get it to work :(
Enter your public IP address (available at sites like this), and press Fish!
If you're on Wayland, or can't get X11 listening, xnest may help. Alternate instructions (thanks Hunter!!):
Run "Xnest :1 -listen tcp" (watch the capital X!)
Run "DISPLAY=:1 xterm"
In the terminal that just opened, run "xhost +"
Make sure port 6001 is forwarded to your machine
Type your public IP, followed by ":1" into the fish box above.