iOS 12 Public Beta could release any time now, so it's best to prepare your iPhone and iPad ahead of time.
Prepare Your iPhone and iPad Right Now if You Wish to Try Out iOS 12 Public Beta
Given how things are, it's likely that Apple would release iOS 12 Public Beta either this week or the next one. Regardless, if you were planning to test out the beta yourself, then it's a good idea to make the necessary preparations ahead of time.
Sign up to the Beta Right Now
Though the public beta isn't available right now, but you can sign yourself to the program right away. Just head over to beta.apple.com and sign in with your Apple ID - the very same Apple ID which you are using on your iPhone or iPad.
Check Device Compatibility
If your device can run iOS 11 then it can run iOS 12 as well. You can check out the complete range of compatible devices by going here: These are All the Devices Compatible with iOS 12 Beta
Download and install iTunes on your PC right away for syncing data, downgrade (if necessary) and creating backups. This will come in handy later on.
You can download iTunes by going to: apple.com/itunes
Prepare to Downgrade if Necessary
There's a chance something might go wrong during usage, so it's best to make necessary arrangements for a downgrade if it comes to that. First and foremost, download the iOS 11.4 firmware file to your PC or Mac and check out the guide posted here for instructions on how to downgrade: How to Downgrade iOS 12 Beta to iOS 11 [Tutorial]
Though you will not lose your files or settings as you install the iOS 12 Public Beta as the update is delivered over the air, but it's a good idea to backup everything. You can either use iTunes or iCloud here. I would suggest an iTunes backup since it is quick and easy.
Wait till the Beta Arrives
If you've done everything mentioned above, then all you have to do is wait for the iOS 12 Public Beta to arrive. If you can't wait at all, then you might want to install the developer beta without a developer account. You can learn more about it here.
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