It was very easy. This was onto my Pi 4 running 64-bit Raspberry Pi OS.
Go to the releases page on the vsCodium website, it’s on GitHub. If you are running 32-bit Pi OS then you need the codium_1.46.0-1591900344_armhf.deb file (the number will most likely have changed). For the 64-bit one, you need the codium_1.46.0-1591900344_arm64.deb.
When you click on the link it will download and ask if you want to keep it. It will then give you the option of opening it and select the archiver. That will install it for you.
After that, to run it from the command line just type codium.
Or as I just discovered. Look on the pull down menu, go onto accessories and you should see (probably near the bottom) VSCodium. Move the mouse over and right-click.You’ll see a popup menu with Add to Desktop and Properties. If you click the former, you get an icon on your desktop.
Screenshots were captured with the Flameshot utility. It’s installed with sudo apt install flameshot. That’s the VSCodium and Flameshot icons.