How do I debug React Native native module in android studio?

Looking for an answer I came across this post: https://stackoverflow.com/a/38007808
However I don't know how to...

launch the React Native packager beforehand

as Jean Regisser suggest to do before in order to start debugging.

This is what I tried so far:

1. Go to android studio and press play button
2. Run react-native start and press play button fromm android studio
3. Run gradlew clean --> react-native start --> press play

Nothing works. Help!