To use Reactotron, just start it (double-clicking should do the job) and you'll get the initial screen shown in the following screenshot. The tool will just wait for your app to connect; sometimes, it may take more than one attempt to get the initial connection started, but after that, things should move along swimmingly:
After you start the application, you will see that it made a connection. Reactotron shows some details: for example, the device is on Android, running version 8.1.0, and we can also see the size and scale of the device. See the following screenshot: ...