If you've been following the React Native ecosystem, or if you investigated for React Native Desktop solutions, you already stumbled into this exceptional fork.
React Native Desktop (for OS X) is already incredibly successfull, mostly because potomushto is doing a constant and impressive work of keeping its project aligned to the mainstream version.

While we waited to cover this onto our community website, he has already published three major versions.
So we're posting this today, one year from the news that introduced React Native to the world (yet to be opensourced later) as an emblem of the long way that React Native has already gone so far.

react-native-desktop on GitHub

Since it's simply a fork of React Native, basically you just can follow the same steps. Feel free to ask anything on #react-native-desktop channel if you run into problems (and you probably will).