Make a boot USB from an ISO file.
Boot the system.
Attach the SSD. Unmount it. (If it’s shown on the desktop, it’s mounted. Right click and unmount it.) Make sure all the disk connections are tight!
Run the Installer in the normal way. When it’s time to choose the disk, pick the external SSD.
Continue with the rest of the install as usual, but skip installing the Multimedia Codecs.
After the installation is done, follow the prompt to reboot.
It should boot from the SSD.
Now you need to install the wifi drivers. In the first boot dialogs, there’s a Driver Manager tool. Run it.
Insert the boot media, and follow the prompts to make it available.
Install the Broadcom drivers (or whatever – it’ll say AirPlay or something like that).
Reboot, removing the USB liveboot thumb drive.
Log in, and then connect to your wifi.
Use first boot dialogs to find the Multimedia Codec installer tool, and run that.
That’s pretty much it.
I like to also install Regolith.
Then, after Regolith is working, I like to install oh-my-zsh.