To enroll your iPhone in the iOS 11.4 public beta, go to the Apple Beta Software Program website and sign up. Back up your device, then download the beta profile and install it on your iPhone. Go to Settings > General > Software Update to download and install the latest beta version of iOS 11.4. Note that beta software may have bugs and can potentially harm your device, so proceed with caution and consider installing it on a secondary device. Make sure to provide feedback to Apple if you encounter any issues.