First edit /etc/apt/sources.list to add contrib, if it's not already there.

Then:

dpkg --add-architecture i386
apt update
apt install steam:i386

I installed the flatpak version and it worked without any tinkering, even Windows games. I've tried the "official" install method (a few years ago now) and ran into problems. I go for flatpak nowadays for simplicity. Lets me have a rock-solid OS and up-to-date apps.