You download all of the appropriate tools to try and get a Android "Hello World" app working.
You create your project with
react-native init HelloWorld and you run it with
react-native run-android and it dies.
I don't know exactly what causes ☝️ but it's easy (albeit completely non-obvious) to fix. Luckily, you only have to follow these steps with a new project.
Create the android assets directory
Manually start the React packager
react-native start --reset-cache
Manually force a build of the android build
curl "http://localhost:8081/index.android.bundle?platform=android" -o "android/app/src/main/assets/index.android.bundle"
Now you should be able to do
react-native run-android to launch your app.
One more thing
If you get any errors about hot-reloading and not connecting to the development console, just run
adb reserve tcp:8081 tcp:8081 and then reload the app on the device.