It's not necessary to use TestFlight to test your app, instead we recommend using debug builds and installing to your device.
If you'd like to use TestFlight, here are the steps.
- Complete all steps for iOS app store submission, including generating certificates and creating your app store listing.
- Add app testers in App Store Connect by visiting your app, click TestFlight, then Internal Groups => Testers.
- Run a "Submit to iOS app store" build on the Build tab of the app customizer.
- When your build is done processing, it should be available to test. You can check the status in App Store Connect under your app => TestFlight => Builds.
All testers will need to download the TestFlight app and enter their login. More information: https://testflight.apple.com/