From upstream releases, choose the file with helix-<version>-source.tar.xz. And run rpmdev-spectool -g helix.spec. Create a directory named helix where you should extract the contents of the downloaded tarball. Thereafter, run osc service disabledrun. This will run cargo vendor and compress them inside a tarball. You can check if you want to disable the cargo update mechanism.

Please do remember to create separate packages for the runtime and the helix binary as the former is huge when built. Helix still runs fine without the runtime but for it to run "normally" a user just needs to run helix --grammar fetch then helix --grammar build. The purpose of the runtime files as a package is to maintain consistency and convenience and stay faithful with how other packagers do it and when someone needs it.