First of all, you need to focus to keep data safe. So you need to take backup on your iCloud or iTunes. Go for any of the preservation methods to prevent any data loss. If any issue occurred then you can go back to the previous version i.e. iOS 11. As you know this is an un-official method so you need to download some files externally.
- First You need a safari browser on your Apple phone where you want to install iOS 12 Beta 1.
- Then open the link (ufile.io/ntrih) on your safari browser.
- In the Page, you will see some link on blue boxes. You need to click on Slow Download.
- Then a popup will appear on your screen, click on Allow to give it setting permission that it needs.
- Now it will lead you to settings where you will see iOS 12 beta Software profile. Click on Install on Top right corner. And Enter your Passcode.
- Then again a page will appear with Install option on Top right corner. When you click on Install it will ask you to Reboot. Reboot manually if it does not Reboot Automatically.
- After successful Reboot Jump Back to settings > General > Software Update. In software update section you will see a normal iOS 12 Developer Beta Update available.
- Then Click on Install Now, it will verify install and then Download the Update. Wait for complete download and installation.
- Now you have successfully Installed iOS 12 Beta 1 on your iOS device without a developer for free.
Source -
https://www.ytechb.com/how-to-install-ios-12-beta-1/ 
06-07-2018, 02:06
Install iOS 12 Beta 1 without PC with third-party method 
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