It is exactly these following steps:
Get the source

   git clone https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs.git
   cd gst-plugins-rs

Build and install:

   meson setup --buildtype=release --default-library=shared --wrap-mode=nodownload --auto-features=disabled -Dspotify=enabled build
   cd build
   ninja
   sudo cp libgstspotify.so /usr/lib64/gstreamer-1.0/

Very simple and straightforward.
Except that it's not. It all goes wrong at the Meson step. Firstly, it required me to get cargo-c. Trying to install cargo-c required libssl-dev, which is not on Solus, and pkg-config, which is not on Solus. After lots of looking, I found the equivalents to install here: pkgconf, and system.devel (which required a -c flag when installing, as well). Given that it has a native Solus version, the instructions should account for dependency issues.

Anyway, after resolving that, I can cargo install cargo-c, and then Meson runs properly. That is, without crashing. Instead, it tells me Run-time dependency gstreamer-1.0 found: NO (tried pkg-config and cmake). That's weird since, putting aside my not having cargo-c already, it's technically just two steps in. I got source from the repo. Went in. Ran the build command. And it's missing. Isn't it one of the very things the repo is supposed to supply? That's not an external dependency, but an internal one, right? What's going on here?